On a A321 MAN-LHR service last August my son and I were walking down the air bridge to board when the TRM appears at the doors and shouted up to "move the passengers from row one to the back".

I identified us as row one pax and he asked us to wait at the bottom of the air bridge until everyone had boarded. He then gave us the a row to ourselves toward the back. No big deal, in the end it turned out well for us as we arrived at a remote stand and 'de-planed' from the rear.

Quite a few times I've experienced the front few rows being blocked on a lightly loaded A321. Those who selected seats a week or so before departure were left, with all the empty seats blocked about at T-72ish.