Dear 'susprised' of Newcastle,
Perfectly possible for the a/c to tip. The A321 (a stretched 320) in particular is vulnerable to sitting on its tail, which is why passengers are boarded via the front of the a/c first, and then through both front and rear doors.
As for the weight of bagage in the hold, it can be quite considerable - just as the weight of passengers is quite considerable. Think about it, 150 or so bags at about 20Kgs each, and 180 passengers at about 80kgs each as a rough example, and you have a load just over 17 tons. If that load is in the wrong place, or starts to move as it would when passengers get on/off the a/c, then that is a sizeable shift in the centre of gravity of the a/c and it only needs to move a fraction behind the main gear for the a/c to act like a giant seesaw.
I would suggest your captain was no fool, and indeed it was rather decent of him to explain the request. My only surprise is that the passengers complied - they normally seem to have extreme difficulty in following the simple instructions issued on arrival, like no smoking, no mobile phones etc...
