Frankfurt - I don't want to get unnecessarily technical, but just to clarify Maude's comment about the APU/GPU, as he says it sounds as though the aircraft was being powered by a Ground Power Unit during the turnaround as the aircraft's Auxiliary Power Unit may have been broken. If the GPU trips off for any reason the aircraft's batteries are the only things powering the aircraft, and can take a big hit if the problem goes unnoticed. This could have been a contributory factor in the turnaround taking so long, i.e. once the problem was noticed the GPU would be reconnected and allowed to charge the batteries, and the Captain may have elected to delay boarding for the batteries to recharge. Additionally, with the APU broken and therefore not providing air conditioning the departure longe is definitely a more comfortable environment than the cabin on warm days.
With a bad GPU the aircraft's batteries would have then taken a further large hit during engine start, so during the taxi phase the crew would not have been waiting for the aircraft to reach adequate power, but for the battery charge rate to reach the required minimum level for take off, as dictated by the aircraft's operating manual.