It is because you are pumping large amounts of power into the txmsn, and the rotor is tilted a long way forward to go fast. This makes the bending forces very high - the txmsn is trying to lean forward but is mostly restrained, the engine cannot move but its drive shaft is being bent by the tranny, and the mast is being bent forward by the rotor.
So, either reduce the forces by going slower with high power, or reduce the power if you want to go faster. Your machine will last much longer if you do so.