Derek had gotten so lost in his conversation and taking care of Erin that he didn't think much when Stiles wandered off with Erin. That is until he could hear Scott and Stiles in the distance. He didn't need his abilities to hear the panic in Stiles' voice.
Without thinking, Derek took off in that direction, whether Cora followed or not he wasn't sure. As he came closer he would come to see Scott running in his direction, with Stiles following behind. Derek might not have been a werewolf for the moment, for whatever reason. But he wasn't going to let his old pack mates get injured.
So, when Erin came into his view in her full form, it would have taken Derek back, but right now he couldn't think that. Aware that he couldn't do much, he did the one thing he could do, he slammed his body full force into the transformed girl.
The impact alone was painful and Derek knew it wouldn't do much but maybe buy some time. "Scott, get out of here!"HE screamed at the young Alpha as Derek did the best he could to keep Erin's focus on him.