IME, the most frequent reason for the rolling delays is late-arriving crew - whether it be from a delayed inbound, stuck in traffic, stuck in customs (whether BS or not), stuck on the toilet. To an extent it's just the human element that you can't always control. Friend had this on DL (regional) the other day, they just didn't know where the pilot was. Had it once on AA - when a FA called in sick and they were able to get a reserve, but they just didn't know exactly how long it was going to take them to get to the gate (I think their contract allows them a certain amount of time if they're called in, they may well have been coming from home). Of course I don't know who wants to arrive at a gate to hoots and hollers from passengers who have been standing there for an hour waiting for you, so you'd think it would be in their interest at least to communicate effectively (of course this could be the airline's fault as well).