Helops - can't see where talkpedlar mentions 'mast-bumping' (maybe he removed it) but it is a condition that can occur in a teetering head helicopter when less than normal G is experienced.
A 'pushover' type manoeuvre, where the cyclic is moved quickly forward, can reduce the G to a point where the main rotor stops influencing the attitude of the fuselage and abnormal angles between the MR and the rotor mast can occur. In relatively benign situations this causes 'mast-bumping' but in extreme cases the MR can impact the tail boom with disastrous results.
Not saying that is what happened in this case but it is a well-documented factor in some Robbie accidents.