I just assume that failing contact of the legs after the knees could be a culprit that i've seen it several times at you(duh,you know).
Because it did same to me.I fixed it by exaggerating the bar drag to the legs even from the floor(my cue)though it hurs my shins,but i tweak the back a lot less than before.
Just my 0.02.Hope you feel better man.