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.
Some things are like paisley ties, they come around every couple of decades. Usurpation has been around longer than most would think, I remember I mentor telling me about some he witnessed in the 1920's. I saw a few back in the 1960's. Had a colony of Caucasian bees in a 12 frame hive that wouldn't build past a single brood box, it was usurped by a swarm from one of my Italian hives. At least 5 lbs of bees. The take over occurred over a 3 day period. It was a very interesting thing to watch as the swarm took over one side of the hive 1st, then the rest of the hive. There wasn't really a lot of fighting between the hive bees and the swsarm and in the end the Caucasian queen was offed and the worker bees absorbed by the swarm, the hive became fairly productive with an Italian queen, building up 2 12 frames deeps and 3 honey supers.