Vega does that everytime I ride her. It doesn't matter if its in the dead of winter, or in the summer, She always does it. Usually just once, but sometimes more than that.
There have been a few times where we've been trotting and bam! She puts on the breaks and down goes her head. I lean back and with one hand hold onto the back of the saddle. (probably not the best to do, but I do it anyways)
I let her do it because if she really has to itch, i'm going to let her itch. She have done it when i'm doing groundwork, so i'm not really sure what it is.
Is there anyway you could ride Willow and when he does it correct him?
Other than that, i'm not sure how to stop it.