Seems likely that the plane was having a problem with the Auxiliary Power Unit (APU). I think in most cases, modern planes push back first, start the APU (which is a mini-jet engine in the tail), use air bleed form the APU to start one of the main engines, and then use air bleed from that engine to start the other engine. (or something like that).

I also think that in some cases an inoperative APU isn't an MEL (minimum equipment list) item. So they start one engine from a cart at the gate and then the second engine after push-back and don't have to take the plane out of service right away.

I'm not aware of any US airport that allows powerback, and haven't seen it done for decades. It's a noise and exhaust management problem for the terminal building and ground crew, and it has a very high risk of flinging foreign objects from the ramp area against the terminal building - or kicking up a dust storm that will then be sucked into the engines.