It all makes sense to me when I think of it this way.
Martinez has been indulging Barkley from the minute he got here in 2013 and Barkley scored that class opener v Norwich I think it was. He's the future of Everton and England, Ross is a diamond, the team is going to be built around him. The media picks up on it, and all of a sudden he's labelled the new Gascoigne. For any kid of 20 years of age as he was then, to be told all that, you could be forgiven for having a touch of arrogance about you.
So, Koeman arrives on the scene and has told him a few home truths (or tough love, call it what you will), he's not the player he thinks he is, he needs to improve, he is no longer a guaranteed starter in the first eleven, he will have to work hard for his place. Barks has no doubt also felt chastened to be dropped on the occasions he was last season. He's been dropped from the England set-up also.
At season-end, he realizes that the "new" Everton will be spending big money over the summer, which will put his place in further jeopardy.
I suspect that it has all come too easy for Ross; and now that he has effectively [Poor language removed] his own bed by not signing a new contract, he is going to find his career may be at a crossroads. I really just can't see a Man U / Spurs / Arsenal guaranteeing him first-team football this season, let alone paying him close to 150K a week (well Man U probably will!).
Even my bias aside, as I'd love Ross to stay, the best thing for him would be to sign the new contract, knuckle down, listen to your coaches, and fight for his place in the current Everton side. I do think he still loves the club - his goal celebration v Burnley hinted at that - I just think it's a ridiculously childish reaction to Koeman's "treatment" of him this past season.