Part of the problem seems to have been the knock-on effects. One aircraft is delayed and so that delays the next and that delays the next and so on. The system is run so close to capacity that there's no breathing space for recovery for when the inevitable problems do occur. Same problems when motorways are busy and one person brakes suddenly - 2 miles back down the road the traffic grinds to a halt.
Not sure what the answer is - leave more time between landings and departures so that there is some slack to allow for recovery? But of course that would cost money!