Once Derek had seen Erin run off into the woods, he would finally fall against one of the trees, from the pain he was keeping hidden. Despite Cora telling him to Run, Derek wouldn't leave his sister. His breathing heavy as he came to look in Cora's direction.
That visible red strain coming through his shirt. It seems when Erin tackled him, she had managed to get a good hit into him. He came to look around, only to see if Stiles and Scott had made it far enough safely. Once they were out of sight, he would look to his sister.
"She needs time to heal... Let's head out of here."