Forgive me if this is wide of the mark but I haven't seen it discussed here..
Is it possible the pilots seat position contributed to the incident? For example if he left it in the position the captin had set it to, and that was a bit far back for him, could that have caused him to in advertantly apply back stick?
At least one problem with this theory is that I don't think the PF was the one that took over from the captain.