from what I've read, Dan Harmon being fired from Community was most likely because he was very controlling, opinionated, and difficult to work with - in particular, Chevy Chase had a very difficult time working with Harmon evidenced by a voicemail recording from Chase ranting about how much he disliked Harmon finding its way to the press... just the same, I have seen an interview with Sarah Silverman explaining why Harmon was fired from her writing staff after the first season of The Sarah Silverman Program - basically saying that while he was very creatively talented, he was also combative, arrogant, stubborn, and all-around difficult to work with

another reason I am sure factored into NBC firing Harmon from Community (for season 4 only, he came back the following season to finish out remainder of series) was that Community just did not get the ratings that a network like NBC was looking for from a sitcom in a huge primetime spot... Harmon is stubborn, but in a good way (at least artistically) as he doesn't pander for ratings and produced a much better show as a result - if you watch Community season 4, without Harmon the show's writing suffers, they over-emphasize love plot lines to appease a broader therefore stupider audience, and they needlessly amp up the sexiness of the female characters