Could be legit #1: Dean Ambrose's career has stalled out at the midcard level
Dean Ambrose has certainly had his ups and downs in the WWE. When the Shield first made their debut on the main roster, they were intended to be a trio of equals.
However, Dean's charisma and low-key delivery quickly made him the defacto leader of the faction, at least in the eyes of fans. He became US Champion and held the title for a very long time, though it has been pointed out that he rarely made title defences. Meanwhile, his Shield mates were the reigning tag team champions.
Fast forward to the Shield being split up, and suddenly it's Roman Reigns and Seth Rollins in the main event picture. Dean seemed to be lost in the shuffle, at least for a time.
However, his ability to resonate with fans and the fact that many of his live events sold out in spite of major names like Roman Reigns and Brock Lesnar not being present led WWE to give him a main event push--for about a hiccup.
Dean lost the WWE Title, and then ended up back in the midcard as IC Champion. One can understand why this would frustrate the Lunatic Fringe and might lead him to legitimately ask for his release.