What happened
During the engine start sequence while being pushed back, both engines of the aircraft increased to a high power setting. This surge caused the airplane to push forward against the tow bar, resulting in a sheared pin and a bent tow bar. The movement also caused the aircraft to strike the tug.
Prior to the engine start, maintenance personnel had performed an automated check that included automatic movements of the throttles and the control wheel. During this test, the flight crew was seated in the cockpit. Once the test concluded, the aircraft was returned to service and maintenance staff exited the cockpit.
The captain began the BEFORE STARTING ENGINES checklist without the pilots first completing a required cockpit flow setup per company procedures. This process was interrupted by a visitor to the cockpit and a non-pertinent conversation lasting 3 1/2 minutes. When the crew resumed the checklist, several items had been missed. During the continuation of the checklist, the first officer called out "THRUST LEVERS" and the captain responded "CLOSED."
As pushback began, the captain verbalized the start of the first engine as required. However, during the start of the first engine, the captain engaged in another non-pertinent conversation with the first officer. The second engine was subsequently started without the verbalization required by company checklist procedures. During this process, neither pilot noticed that the engines were accelerating above normal idle.
The investigation
Follow-up testing determined that the engines accelerated to a power setting corresponding to the final position of the throttles at the conclusion of the automated maintenance test.