Did you check for swarm cells?
They probably were the very first nuc I ever purchased did the same thing and landed 5 ft from the hive they came out of so I plunked them into another box and set then down 1 ft from their original hive they stayed and both hives built out greatI did do some inspection. I added the second because the first was filled to the brim. the strange thing was is the upper box wasn't very full. its almost like they were in swarm mode before I added the second.
It was probably a 4 frame nuc size.