Stiles didn't know what he was doing, how the act of biting Derek's neck was an act of dominance. He just liked it and continued to do it as he continued to rock. His pleasure was building fast, the burning of the stretch was speeding it up and before Derek could take his finger out Stiles was coming undone against the werewolf.
This caused Stiles to really bite down on Derek's neck as he cried out.