Strange question, and strange place to move it!
The only genuine H&S reason I can think of at the moment would be that the aircraft arrived in from a place that has a problem with some form of illness or infection, and a passenger was taken ill, or similar, so the aircraft has been taken out of service to be appropriately fumigated or whatever.
Having said that, on a similar vein, if something like a rat or mouse or other animal got into the aircraft and was seen, the aircraft is probably not going nowhere till the remains have been found, or it's been caught, or seen escaping from the aircraft, and even then, there will be some inspections to be done.
There should be no issues on the catering side, as almost everything in the galley is removed after each rotation.
This would be more of a tech issue than an H & S, but they might have chosen to call it H & S if there was a major problem with aircraft toilets blocked or otherwise unservicable, that's another possible reason for cancelling a flight.
Other than that, someone might have used H & S as a "cover" for crew non availability, by saying that they had to have rest, but that's not really a true H & S issue, that's flying regulations.