From an April 6 New York Times article:

The negotiations that led Glenn Beck to announce his departure from the Fox News Channel on Wednesday ended with an expression of “let's part as friends,” according to several people with knowledge of the talks. But behind that moment was a torrent of acrimony that underscored just how fractious the relationship between Mr. Beck and the network had become during his three-year run on Fox.

[...]

Beck supporters presented a picture of constant sniping, planted stories about his declining ratings, and discomfort with his ability to build a career for himself outside the Fox News brand.

From Fox's perspective, the facts about Mr. Beck's run on the network have been public and indisputable. Among those were the refusal of hundreds of Fox advertisers to allow their commercials to be placed on Mr. Beck's program, and a history of incendiary comments that attracted harsh backlash, including one where the host called President Obama a racist and another where he compared Reform Judaism to radical Islam. (He later apologized for both comments.)

Mr. Ailes suggested in the interview Wednesday that he was happy with the departure being characterized as either a cancellation or a decision by Mr. Beck to quit. Fox has retained all its other high-rated hosts in the past, but they didn't come under the intense scrutiny that Mr. Beck has faced, nor the mass opposition from advertisers.

[...]

At times, Mr. Beck and his managers said they sensed that Fox was retaliating in public, although they did not prove it.