Originally Posted by BoeingBoy

While technically unrelated to the air start, many jet bridges have safety features built in to prevent moving them until the external electrical power is unplugged from the aircraft and the cord retracted. This is to prevent damage to the aircraft that could result if the jetway were moved back with the cord still connected.

When an air start is required because the APU is inoperative (not just the bleed air), the external power cord is left hooked up while at least one engine is started and provide electrical power. Therefore many jet bridges won't move until after starting the first engine, switching electrical power over to the engine-driven generator, and disconnecting/retracting the external electrical power cord.

Jim