Will be interesting to see if Liam is thinking the same as me, but picture the hamstrings pulling the hips forward, not flexing the knee (well, maybe a little of that as well). Todd's comment is correct, it's easier for the upper body to pull the legs back than for the lower body to pull the hips and torso forward.
http://harbskisysems.blogspot.com/2012/04/blog-post.html