I feel like he wanted to pull the classic Z move and put his hand in the front pocket but couldn't because of the clothing label there which I'm assuming they wanted photographed since they were a party sponsor. So he had to use the butt pocket. This awkwardness is making me lol slightly.
Hopefully JJ will post more of Zac cause there are more on wireimage.
1: jj (which has a couple more of Brittany and Ryan), 2