He's suffered a dip in his confidence, owing primarily to some cold shooting, which has in turn led him to bring less than 100% effort and focus on every possession and at both ends of the floor. He has allowed a few bad calls and other moments of frustration to affect his demeanor way too much.
This, as we know, is the easiest route to Coach K's (temporary) doghouse. Long-term, he will be fine, but for now, he's (probably rightfully) sitting and thinking a little bit, while watching others (namely, Seth Curry) parlay solid practice and continuous resolve into starter's minutes and strong contributions to the team.