I was thinking that myself Mycroft.
There is a crew shortage at the moment, so I am thinking that there was no crew to hand (MAN staff are being re-deployed to LHR) so replacement crew would have to have been sourced from LHR, which would have at least delayed the flight. This would have thrown out the operation and left LHR short of standby crew. If they could have re-routed the affected passengers, this may have been the better solution at the time.
It certainly wouldn't have been an easy decision and would have been decided by a number of people considering a number of factors.