The master and gen buz bars on inspection are on the passenger side with a sprung loaded bar that can switch off all three at the same time (now being modified).
The headsets in use at the time had thick pads across the top of the head very close to these switches but under normal circumstances not a factor.
Fly along at cruise torque with not so tight seat belts and slam the collective down (not recommended). Doesn't take too much negative G to lift you a couple of inches out of your seat. (Done it in my wild and crazy days in a BO 105, settled down now though).
Before the real reason came to light, they thought the passenger bent foreward, leaned back and switched everything off causing a sudden loss of SAS and loss of instruments. Only thing they could come up with at the time, even though he would have had to lift his bum at the same time. Flat pitching would lift your bum just enough to reach those switches and that is what happens when that part breaks!