Just spit balling on why he could possibly want out.
Besides feeling like he's underpaid:
He might dislike Indianapolis (the city).
He might be resentful how we've kind of ridden him hard so early in his career (about 800 carries in 3 years).
Or he possibly got fed up waiting for the org to fix the QB position. Maybe a to little to late type feeling.
Who knows, but definitely looks like he wants out HARD.