I remember reading something once by a guy who was annoyed to hear rich people talk about the days when they were poor. He thought they were bragging. But I find I do this all the time, and mostly not to anyone else. I do it to fight the pull of the hedonic treadmill-- to try to get my old poor self back so that I can appreciate not being poor anymore.
Possibly some of the people he thinks are humblebragging are doing something similar.