On one occasion at BHX, a Flybe flight was 'on time' until 'go to gate' time arrived, then suddenly a two hour delay appeared. As this was an internal flight, they would have known hours before then. A sternly worded letter at produced some compensation though!
Could be down to a last minute aircraft change to protect the departure time of another flight. Often see airlines deciding to take the hit on one flight to protect another (usually down to crew hours, airport curfews etc)