I don't know what the algorithm is but I've been in Row 2 of an A320 and was bumped back in to Y when there was an a/c swap to an A319. I don't think that avoiding the last row will necessarily protect you from anything. It certainly didn't protect me. There was even an open F seat but the Gate Agent refused to give it to me because he said that he had a paying F customer on the upgrade list. I had upgraded a month earlier using an RPU and had chosen that route because I could upgrade it using an RPU.

In any case, I do avoid the back row of F if possible because it often has limited recline