BK pitch-up
I have seen the pitch-up on an almost regular basis when above about 10,000 DA and at high AUW and pushing the machine along at close to VNE for the conditions, but never seen it at low DA, AUW, or speeds.
I have always assumed that the pitch up was being caused by retreating blade stall. Lowering the collective certainly helps correct the pitch up which is consistant with the recovery technique for retreating blade stall as well.