There is also the theory that the constant ignoring of Ross by England may have also led to him questioning himself.
Before the last tournament, he had played in a lot of the qualifiers and seemed to be making himself a regular. The Jack Wilshire was brought back in ahead of him and all of a sudden he was ousted and sent further down the pecking order.
Since then he has basically been shunned by the national side, then Koeman has spoken out about him, hauled him off at half time in a game last season etc....so it would be no surprise to me if he was feeling like things could have been turning against him at Everton as Koeman is certainly not the sort of guy to pander to anybodys needs.