My theory is a combination of reasons.
1) First & foremost his confidence has been Yo-Yo'd.
Remember this?
http://forwardcenter.net/lillard-uses-top-5-point-guard-omission-as-motivation/
Snubbed from the list of Top 5 (really Top 9) PG's. It clearly bothered him. But then he goes out and has 2 of the best games of his career against SA & OKC in the same week. The narrative changes. Less than a month later it's "Who's better: Lillard or Paul"
http://insider.espn.go.com/nba/story/_/id/12165319/damian-lillard-chris-paul CP3 outplayed Dame that night, but he also got all the calls . . . in a game @ PDX. No respect! That was the night his shooting slump began. He's still having a career year up to this point. But 2 weeks later he's snubbed by the coaches for the AS game. That hurts bad given the kind of year he's having, but he's still going to play because Kobe's out. "Not the way I wanted it . . . " he said. Then, the unthinkable - Silver picks another player from a losing team who's led the league in tech fouls 3 years running (head case), & missed a large chunk of games with viral meningitis. Dame has yet to miss a game, EVER. Cousins is nice for sure. But wow, really? Does anybody think I'm worthy? Self doubt creeps in. Not only that, but the pressure of his first signature shoe launching with a special colorway to be showcased at the AS Game (that he won't be playing in!) He ends up "making" the All Star team thanks to a freak staph infection to BG. But does he really feel like "fits in" as an All Star? Not to mention the actual game was all about his nemesis, Russ Westbrook (these guys don't like each other - Dame respects Russ, Russ resents Lillard's "Most Clutch" label & thinks Dame isn't even in his league) One more thing - injuries to his teammates are piling up & the team is struggling. A lot of eyes are expecting a bitter for being snubbed #O to save the day. We don't think about how these things can affect a player, like they should be able to block all that stuff out. He's human. He'll bounce back, but his swag is currently MIA.
2) Injuries, Batum's struggles, & the way he's being defended.
Teams have been able to smother Dame defensively w/o having to pay a price for giving him extra attention. Batum has . . . enough has been pointed out here by others. Not bashing Nic, but his poor play has given opposing teams an extra incentive to force the ball out of Dame's hands. There simply hasn't been a consequence for leaving Nic open or allowing him space on the perimeter. And as good as Wesley is as a "catch & shoot" guy, he's simply no threat at all off the dribble. Now factor in all of the injuries and line-up changes & even the injuries Dame sustained himself. Sprained fingers, sprained ankle, bruised hip bone . . . and still hasn't missed a game.
I think, no, I KNOW there will be "Dame Time" again. This is just a bump in the road. I'm more worried about how the team is going to get better than I am Lillard getting back on track.