Whilst those two reasons can often be the case - and I don't see any issues with the first one in any case - they're far from the only ones.
It's frequently just to take the wind out of the sails of the boorish/stalky self-superior people who see a username they don't recall, then they AS the name, and say "Interesting first post, OP........" in a clear way to suggest that they're a troll or a WUM.
Of course, the people could indeed be trolls - but on the other hand, posters do frequently genuinely use MN for the very first time, and may well want to discuss an embarrassing or awkward problem that they don't feel able to raise with family or friends as their catalyst to becoming a regular.
Also, it's not necessarily something bad/outrageous that the OP has said or done. You do get bullies on here who fixate on and set about somebody for no good reason; so name-changing is like the online equivalent of walking a different way home from school where you won't get beaten up.
The point is that identifying an unfamiliar username without any history is no proof whatsoever of a time-waster, fantasist or troll and isn't the automatic gotcha that some people assume.