We had a friend break his collarbone this May on a descent down a mountain on the Blue Ridge Parkway. He is a really tall person on a large frame. His bike shook violently and he crashed.
Examination of causes is that it is believed the front wheel was not tightened (clamped/skewered) properly that morning and this was the first downhill of the day. The wheel was mis-aligned.
He exacerbated the problem by unclipping. We had not heard of the "press knee into frame" at that time but I now preach it to fellow riders when we are doing hills and to have them double check their front wheel as well.
Other posters to this problem mention that poor tolerances (out of quality spec.) front forks can cause this problem and this makes sense to me.
Might want to check your front fork. I would advise to get this fixed or disaster could occur in the future. I would not ride this bike on a hill until fixed.
Regards, curtwally