I don't watch him much when he is outside in his pool, but it seems like 75% of the time when they get treats in the water he will do this. It's very odd. When I pick him up he will also do something similar and push his neck outwards unless he has my hands under his feet.
Personally, if its mostly when they're being fed treats, and overall he seems healthy besides that, I'd just leave him alone. I do have some birds here, that do similar stuff.
His WHOLE body vibrates sometimes but I feel like he only does that 1/50 times when I pick him up. (either he's happy or scared! Probably happy, he's extremely tame... at least that's what I've read about ducks that vibrate sometimes. Lol.)