The aircraft was parked on Stand 16 and the flight crew was preparing to start the engines using an external ground air supply (aircart) because the aircraft's Auxiliary Power Unit was unserviceable.
The aircart was reluctant to start and this introduced a delay.
http://www.aaib.dft.gov.uk/cms_resources/dft_avsafety_pdf_02...

However, if a problem is discovered just before or during the engine start procedure, a delay like the one you experienced may occur while an external air cart is located. Normally just one engine will be started with an external source, after which external air will be disconnected. The other engine(s) is then started using the "cross bleed start" procedure, meaning one engine is used to start the other engine (s), sometimes during the taxi for takeoff and sometimes while just off the gate. Again, this is a simple procedure and is more expeditious than being tied to the gate, so to speak, hooked up to an air cart.
http://www.usatoday.com/travel/columnist/getline/2006-04-10-...
While at the gate, either an external air cart or the APU (Auxiliary Power Unit) are used to power the aircrafts air conditioning units. We call them air conditioning packs. Just prior to pushback, we’ll disconnect the external cart and transfer this function to the APU. An APU is a supplementary on board jet engine that supplies air and electricity when the main engines aren’t running.
http://www.stickandrudderaviation.com/