Usurpation seems to be a recent topic. Dwight Magnum has written at lest one article on the subject. I've observed it a few times. When it's happening it looks like a cross between a swarm and robbing. The swarm seems to be moving into an occupied hive, but the hive is often fighting with the bees from the swarm. Queenlessness does seem to be an added trigger. I have watched these in progress in Massachusetts, Nebraska and Arizona. I've also seen where it seems that one nuc, going into winter, has moved in with another nuc going into winter, but I did not see it actually happen. In these cases, all I know is the nuc box that had a small colony in it is empty and the nuc next door has a lot more bees.
My guess is that this happens a lot and has been happening for a long time, but has only recently been noticed or documented. It is often blamed on African genetics, but the VA (Dwight), NE and MA ones were all EHB.