That the wall studs were not completely dry when the drywall was installed. Over time, the new house dried out and the studs "shrunk" This effectively was the same as the screws being pushed out, even though they didn't move. The wood shrunk from around them.
I also suspect the waviness in the drywall may be due to the studs warping as they dried out.
In my opinion,your contractor should stand behind the work and be responsible for the repairs. Don't delay, the house will soon be 1 year old, and that may be an excuse not to do repairs if your "warrany" has run out.