I found this article from The Daily Beast which  lays out 11 theories as to why Sarah Palin might have quit:

By The Daily Beast

Sarah Palin’s holiday-eve decision to resign as governor blindsided TV commentators and bloggers who scrambled to come up with an explanation. The Daily Beast’s presents a roundup of insta-reactions.Plus: Read more of The Daily Beast’s coverage of Palin’s resignation and the GOP implosion.

THEORY 1: VANITY FAIR HURT HER FEELINGS
Palin locks horns with the media on a regular basis, and the most recent addition to America’s Palin-bashing oeuvre—Todd Purdum’s 10,000-word Vanity Fair profile—was a doozy. Palin’s closest colleagues ridiculed her with nicknames like “Little Shop of Horrors,” accusations of incompetence and a personalityprompted Palin to ask for the president to intervene), Talking Points Memo’s Josh Marshall characterized Palin’s resignation speech as “a colossal sulk… an effort on her part to ingeniously combine anti-liberal media-bias agitation with Christianist politics by portraying herself as having been crucified by the liberal media.” disorder, and leaked personal emails. Alluding to Palin’s notoriously thin skin when it comes to media criticism (last week, an amateur blogger named “Celtic Diva”

And really, what better way to screw the media than making such a huge announcement on a holiday weekend? Gripes Politico’s Ben Smith: “So much for today being a slow news day.”

THEORY 2: SHE’S RUNNING FOR PRESIDENT
When Palin first announced she would not seek re-election, the conventional wisdom was that she would be running for president in 2012. As recently as last week, the governor was fundraising aggressively for her political action committee, SarahPAC. Early Palin adopter Bill Kristol wrote on The Weekly Standard’s blog that, if Palin’s still aiming for the White House, resignation is “an enormous gamble—but it could be a shrewd one” so she can work on her presidential chops without neglecting state duties:

And haven’t conservatives been lamenting the lack of a national leader? Well, now she’ll try to be that. She may not succeed. Everything rests on her talents, and on her performance. She’ll be under intense and hostile scrutiny, and she’ll have to perform well.

Barack Obama never finished his first term as senator—maybe it’s a new trend? The Nation’s John Nichols considers a “gonzo-style populist run against… the entire political establishment”:

And her talk about supporting change agents inside the Republican party and “outside it” suggests that she might actually be open to an independent or third-party bid.

THEORY 3: SHE’S NOT RUNNING FOR PRESIDENT
MSNBC’s Andrea Mitchell floated the notion that Palin isn’t aiming for the White House or any other office. She genuinely hates politics, Mitchell argued on television moments after Palin’s speech:

She has told some of her biggest backers that she is free to choose other candidates for 2012… She told some of her top backers that she’d had it and was out of politics. She does not want to seek elected office… Sarah Palin will remain unpredictable but… as far as we know she will not be running for president.

THEORY 4: SHE’S PREGNANT AGAIN
Shortly after Palin’s speech, a flummoxed Rick Sanchez started throwing out possibilities, to correspondent Candy Crowley’s chagrin:

Sanchez: There have been a couple of other, ah, situations that might cause someone to feel a lot of stress… and the one thing that’s still left out there is, Hey! Could she be pregnant again?
Crowley: Well, I certainly don’t know the answer to that last thought.

THEORY 5: SHE’S SICK OF ALASKA
CNN’s John King reported that a source close to Palin’s political team said the resignation was a “calculation” related to her “book deal and other issues” that were “causing a lot of friction up” in Alaska. In her resignation speech, Palin said she can do as much good for Alaskans outside of office as she can in it—but is it actually the lower 48 she’s after? CNN reports:

Another source, speaking on the condition of anonymity, said that Palin “thinks she has accomplished goals she has set forward. … She sees what a positive influence she has had on people’s lives from traveling the country in the last year.

Former Palin campaign manager Laura Chase thinks Palin’s taste of the high life ruined Alaska for her:

Out there she’s governor–almost president-elect…. They introduce her with pomp and circumstance. Build her ego up, do that whole thing. Here, she comes back, she runs into a bunch of Alaskans. It’s humbling…. She’s just one of us. But she decided she wasn’t going to be one of us.

THEORY 6: IT WAS A WHIM
Talking Points Memo notes that in the moments before her resignation speech, Palin’s Twitter may have suggested that she hadn’t decided yet on resignation. Palin’s blithe Tweet referred to her decision not to seek reelection, but nothing else:

We’ll soon attach info on decision to not seek reelection… this is in Alaska’s best interest, my family’s happy… it is good, stay tuned.

Politico’s Glenn Thrush notes that Palin met with colleagues all week and “gave no indication” that she was quitting.

THEORY 7: THE ETHICS POLICE WERE ABOUT TO NAB HER
In her resignation speech, Palin said she refused to stand by while Alaska wasted taxpayer money investigating her, presumably referring to the state’s ethical inquiries into Palin’s behavior in the so-called Troopergate scandal. Earlier this week, the Anchorage Daily News reported that the bill for the Troopergate inquiry had neared $300,000; on the same day as Palin’s resignation, the ADN reported that Alaska’s Department of Health and Human Services Chairwoman, Beverly Wooley, says Palin unfairly forced her out of office because “Palin felt she wasn’t in step on social issues.” Meanwhile, The Daily Beast’s Max Blumenthal reports that Todd Palin’s snowmobile sponsor, Spenard Building Supplies, may have been mixed up in questionable political and Palin family business.

THEORY 8: SHE’S UNSTABLE
Democratic National Committee spokesman Brad Woodhouse issued a statement characterizing Palin’s resignation as “bizarre behavior”:

Either Sarah Palin is leaving the people of Alaska high and dry to pursue her longshot national political ambitions or she simply can’t handle the job now that her popularity has dimmed and oil revenues are down. Either way, her decision to abandon her post and the people of Alaska who elected her continues a pattern of bizarre behavior that more than anything else may explain the decision she made today.

THEORY 9: HER HANDLERS ARE INCOMPETENT
Could Palin’s resignation shocker simply be the product of poor handling? Republican strategist and CNN contributor Ed Rollins said Palin looks “terribly inept”:

You don’t just quit with a year and a half to go. You certainly don’t do this as a stepping stone to run for president. You finish the job that you’re in, and obviously she’s not doing that…. You don’t quit on the Friday of a three-day holiday. If you are going to do this, you think it through, you give a good speech

The Washington Post’s Ezra Klein argues that novice slip-ups in Palin’s transcript suggests the soon-to-be former governor wrote the speech herself:

The proof is in the punctuation…. The style is closer to a high schooler’s angry diary entry than to an official speech. I’ve read a lot of speech transcripts. They tend to have fewer words in all capital letters…. today’s speech wasn’t the carefully vetted product of the team quietly masterminding her presidential run.

THEORY 10: SHE’S ANGLING FOR A TV SHOW
Longtime Palin detractor Andrew Sullivan burst into a series of “stunned… yet not surprised at all” posts on his blog at The Atlantic, and notes that she could always pull a Huckabee and parlay her political cred into a career on the small screen:

Once the klieglights hit, it was only a matter of time before she imploded or exploded or some gruesome combination of the two. The librul media will be blamed for everything on her inexorable path to becoming a Fox News celebrity. Maybe a reality show? Someone hire her for The View!

THEORY 11: SHE HAS A GOD COMPLEX
In his controversial Vanity Fair profile, Purdum noted that multiple Alaskans who worked closely with Palin accused her of having narcissistic personality disorder. While some argued that Palin’s attention-seeking qualities were de rigueur for politics, the governor’s resignation speech suggested a desire to avoid a drop in relevance during her lame-duck session:

Once I decided not to run for reelection, I also felt that to embrace the conventional ‘lame duck’ status in this particular climate would just be another dose of ‘politics as usual,’ something I campaigned against and will always oppose.

Sarah giveth and she taketh away.