The 757 is 235 in high density, the A321 Neo is 240 for some airlines.
The 75 was see likely to tip because it disembarked fro. door 2, so it emptied from the middle first. The 321 is a doors 1 plane, so the front cabin empties first. Put that together with an aft loading for bags and you can get into trouble if the baggage handlers aren't on their game.
I have to say I never thought it would actually go though!
I remember years ago, sitting on the flight deck and all the light came on and the ECAM went nuts. The aircraft had pitched up enough to set off the ground/air logic. The captain ran out side and low and behold they had started unloading the front hold first.