What's annoying about "operational reasons"?
Alternatives are "technical reasons" or "weather".
It is then up to the speaker to decide whether to amplify on these and risk offering too much information, an easy trap to fall into as the travelling public are equally as eager to read things into what you didn't actually say as they are ignorant about the complexity of airline operations. Say "For technical reasons, actually they had a hydraulic leak" and a proportion of the pax will freak out thinking they're shortly going to die and another few will be spreading alarm and despondency by loudly suggestin the engineers will just bodge the repair until it gets back to base because everyone knows that's what airline mechanics do, if they even actually found out what's really wrong. "Are you telling us there's something wrong with the x/y/z and you still expect us to get on that plane and fly on it?" or "How do we know they fixed it properly?" is the sort of nonsense all too commonly heard. Sometimes the less said the better. Otherwise you'll get in a converssation with some smart alec keen to demonstrate his superior knowledge of the subject in question and he's out to make a point. Believe me, this really happens!
Equally fudging the issue and especially not telling the truth is hazardous too. It's a skill that takes some learning, like filling in reports; generally the less said the less there is to hang you with but don't hide significant information.
If staff don't ahve the skill, information or ability to say the right thing it's best they say s little as possible, annoying as it may be to those who want to know more.
The thing pax so often seem to forget is that the airline isn't doing a delay for fun, to annoy them or (usually) because of their incompetence. They hate delays as much as the pax and the slow turning of the wheels is a sign of the complexity of an airline/airport operation, not sloth or disinterest.