From what I've heard on the ground a couple of things have actually happened.
The first being that the airport did know how many passengers they were expecting at the time but a large number of staff are now isolating due to a Covid outbreak, meaning that even with the best will in the world there will only be a finate number of staff available to work, which when your available workforce is decimated by 40% then any industry will suffer.
On top of this a large number of passengers (not confirmed, just opinion from someone on the ground), are now arriving 1-1.5 hours before departure and suddenly being caught out.
I guess it doesn't help when flight departures are bunched together in waves but I believe that passengers are advised to arrive earlier than they normally would for exactly this reason. I know the several times I've flown in the past year I've arrived 3 hours early, a couple of times being caught up in big queues but also on occasion flying through.....