When Don catches his ankle, he smiles. That 'a boy, now you're thinking.

Raph hobbles a moment with his leg caught, trying to twist himself out of his grip before falling over. But then it becomes very clear that he is falling whether he likes it or not. So he rolls with it, hopping a little on his other leg and using it to tangle with Don's as he gives into gravity.

If he's going down, he's gonna make sure Don's coming with him.