From the examples the people I mentioned above came up with, guilt seems to be a very strong trigger of rationalisation.

Perhaps rationalization is an adaptation that develops when people risk some kind of punishment for their irrationality.

We are irrational, and we already suffer the consequences of our irrationality. But if there is additional penalty for admitting irrationality, it gives us incentive to pretend that the irrational decision was in fact rational; to lie to others, and ultimately to lie to ourselves. Admitting irrationality can be a very bad signalling.

How exactly does guilt become a part of the equation? Probably by believing that there is no such thing as irrationality, and people are always perfectly following their utility function. So if you forgot to do something, it means you decided not to do it, because it gives you negative utility. So whenever your irrationality harms people around you, it means you hate them. (If your irrationality sometimes harms you, this can be explained away by saying that you didn't really care about something, only pretended it.) From the outside view, our irrationality is not credible -- it may be just a public act, while we are following our true preferences (defined circularly as "that what we are following", plus some possible secrets).