Sounds like a case of passengers were sent to the aircraft and allowed up the steps before the crew had given the all clear to the ground staff. Probably no1 there, or pointless to take them back to the terminal, so were just held on steps.
In my experience at CWL the usual procedure is steps on and engineer or pilots present. No pilots/engineer, no boarding.
Should there be a reason for re-fuelling, or a top up once all passengers are on, and a/c is ready to go. Then 1 set of steps at front, and doors armed at rear.