I suspect that your aircraft had an inoperative A.P.U.
The A.P.U. is a small gas turbine in the tail of the 737 used to provide electricity and compressed air on the ground.
The starters on the main engines of the 737 are powered by compressed air. Without an A.P.U. a supply of compressed air would be supplied from a ground cart. Electricity would be supplied from another cart.
One engine would then be started "on the gate" the ground air supply and electricity would then be removed, the aircraft would be pushed back and the crew would start the other engine useing air supplied from the running engine. This is called a cross bleed start. This is the method you would use to start an engine in the air too.
If you were to suffer a DOUBLE engine failiure, you would then have to re-start the engines by useing the airflow through the engines to do a"Windmill start"!.
The whole "Airstart" proceedure is a pain in the A**e for the ground crews so we try to fix the A.P.U's as quick as possible!!.
Rgds Dr. I.