Event Description
During pushback operations, the airplane was observed to "bounce" up and down at both the nose and the tail before the nose gear collapsed.
Aircraft Examination
Examination of the aircraft revealed a broken lower drag brace that was buckled in compression and fractured due to overstress.
Tug Examination
The tug was examined and found to have a defective gear shift lever. The lever would not lock in the neutral gate and could be moved easily through the gate between forward and reverse gears. Additionally, the tug experienced "hesitation and hard jerks" when shifting from forward to reverse.
Scene Evidence
Skid marks at the scene were the same width as the tug's wheelbase and were consistent with sudden engagement of the transmission.
Conclusion
The combination of the tug's transmission defect and the resulting sudden engagement likely led to the forces that caused the nose gear collapse.