What happened
After being pushed back from the gate, the crew successfully started the #1 engine. During the subsequent start sequence for the #2 engine, the flight crew noted high fuel flow. Simultaneously, a passenger seated on the right side of the aircraft observed a flame trailing behind the exhaust.
Following the observation of the flame, the passenger shouted "fire," opened the #2L door, and deployed the emergency escape slide. This action triggered an uncommanded evacuation of the aircraft. Upon being notified by a flight attendant that the remaining passengers intended to evacuate, the captain ordered the #1L and #1R doors to be opened and the emergency slides to be utilized. The captain then secured the engines and entered the cabin, at which point the evacuation of the remaining 24 people was halted.
Findings
The investigation noted that a high fuel flow occurred during the engine start process, which was accompanied by visible flames aft of the exhaust.