Sarah Palin's increasingly testy relationship with Fox News took a turn for the worse this week, when she wrote on her Facebook page that the cable news channel had canceled interviews she had been scheduled to give during the network's coverage of the Republican convention. Palin was supposed to appear on Fox on Wednesday, the same night that Paul Ryan, her successor as the GOP's vice-presidential candidate, was speaking. Fox executives say Palin got cut simply because time was tight after the convention was condensed from four days to three in the wake of Hurricane Isaac. But this is far from the first public clash between the former Alaska governor and her employer. What's the real story behind the souring relationship? Here, three theories:

1. Palin is not the star she was when Fox hired her

Four years ago, Palin was the darling of the GOP, says Scott Paulson at Examiner.com, but she threw that away when she quit her job as governor of Alaska. She's "not even a politician anymore." Face it: With her relevance in the Republican Party fading, Palin's "extended 15 minutes" of fame are running out. And fame is what made her valuable to Fox.

2. And her ratings don't justify her monster salary

This is "a classic display of Sarah Palin being, well, Sarah Palin," says Gabriel Sherman at New York. But her Facebook outburst also "reveals something deeper" about Palin's rocky relationship with Fox. Palin is the cable news channel's highest-paid contributor at $1 million a year, but Fox executives are disappointed with her ratings. Her contract is up in January, and Fox is weighing what kind of renewal to sign, "if they sign one at all." Palin, disappointed that she's not getting top billing, is pushing back, but she shouldn't press her luck. Since she quit as Alaska's governor, Fox has been her soapbox, and if she loses "the network's platform, it's unclear how she could maintain even her current, much-diminished level of visibility."

3. When it comes to Palin and Roger Ailes, it's personal

Fox News chief Roger Ailes isn't crazy about Palin, says Jack Mirkinson at The Huffington Post. He once reportedly said the only reason he hired her in the first place was that she was "hot and got ratings." He also has been quoted as saying that she had "no chance of being president," and was reportedly furious when she announced her decision not to run for the 2012 nomination on a conservative radio talk show instead of on Fox. In private, Ailes has even reportedly called Palin just plain "stupid." Ouch.