Then I can only conclude that this was a deliberate action by the cabin crew in some kind of a labour dispute.
TLB. Want to put a wager on that?
At my gaff its pretty standard that after any thing like the above the crew go to the hospital for a check up. At BA that's even more likely so. There are three questions;
1. What was it.
2. Why deplane with the punters onboard
3. Why just the Cabin Crew (you have to assume crew rest area)