Could have been a number of things there, however, you are right that this is against all SOPS; as the standard is two gate staff and a despatcher on this type of flight, at the time of boarding i would imagine one gate staff at the front of the lounge, one at the end of the lounge checking seat row numbers and the despatcher would have been between the bottom of the stairs and the aircraft marshalling passengers.
As the lounge faces the aircraft and you could see steps on the aircraft and not connected to an airbridge, i'd like to think all the other passengers were as switched on as you; did the boarding announcement state for passengers to go down the stairs? either way, as per the other reply about rounding up cats, i liken it to sheep, and it would only need one passenger to not listen to the announcement and open the airbridge door instead of going down the stairs and other sheep will follow!
Unfortunate but does happen but shouldn't have