Let's stop blaming 25-minute turnrounds.
We did those in the '70s, BAC 1-11 and B737, as a matter of routine, allocated seating, lots of bags, manual loadsheet, LMCs, and usually fuelling to next sector minimum and-a-liddle-bit. It was not thought to be onerous, and it wasn't.
And we did not corral departing passengers for 30 minutes at the gate so that they could be shoved aboard in the minimum time.
There's clearly lots of Human Factors to look at when two well-trained and competent teams (flight deck and ground) make an error like that, and I sincerely hope that the incident will be investigated using MEDA by an expert to see what the real root cause(s) was(were) and not just write it off to "pressure". Because if the real cause is not found it will happen again, and again, and again.........
If I were doing the investigation, I would start with a very open mind, and look for more than one cause.