I've done this on my steel darning foot a couple of times. It usually happens when I just starting after a full stop. It is just me moving my fabric too fast before the machine gets up to speed. You don't have to tug very hard to pull the needle to the side enough for it to hit the foot. If you happen to be pulling when the needle is up - wham!
Now I try to make the effort to get the machine up to speed before moving the fabric.
BTW, have you modified your foot like Leah Day does? It really helps for visibility. Check this link.
http://www.youtube.com/watch?v=APD6s7PwoqU