Airlines have tech delays. I've just been looking at a 17hr delay Thomson Airways. Monarch have the best engineers in the world in my opinion but sometimes airlines have tech delays and if it is a busy period they often cannot swap aircraft especially when crew hours come into the mix etc.
Some other delays from LGW today:
EZS 8481 Geneva delayed 2hrs 35mins
TCX1523 Dalaman delayed 3hrs 25mins
It's the nature of the business. If the early departure is hit with a delay in the summer it normally means it knocks onto other departures of the day. No airline wants a delay. Delays = £££