Cora had heard what was going on with Erin long before Derek had. She should have gone with Stiles, not that he should have been there. Derek either. They were both human and should have been home and far, far away from here.
She had seen Stiles first and then Scott right on his tail. Before she could do or say anything Derek decided to be stupid and stop Erin from going after Scott and Stiles.
She took off to the left. She wasn't going to come straight at Erin, not when Derek was in the way. Instead, she made a run towards Erin at an angle and tackled the werewolf away from Derek.