I'm an admitted cynic, but there are more possibilities.
-- Off-camera friction between EH and one of the producers or one of the cohosts which led to a showdown ("This morning show ain't big enough for the both of us..." etc)
-- The show/network poised to become the mouthpiece of a specific candidate or agenda that doesn't align with EH's politics (there is a history of that happened at Foxnews)
-- The creative team behind the show decides EH skews towards the wrong demographic (or they didn't get some of those big "The View" numbers), but they also don't want to alienate them like The Today Show seems to do with every personality change, so they spin her departure for the public
-- She's lined up a deal with another network, but no doubt Ailes has her in a non-compete clause so she's getting out now, will write one of those wretched Ann Coulter type of books to stay in the public eye, then become co-host of CBS & Friends in 2018 or something
She worked on a morning show. By definition that leaves her most of the day to be with her family. Citing family reasons just doesn't wash. All she's presently missing is breakfast.