My guess is washboarding. I've watched mine change from lining up and moving in unison to individuals doing their own thing. I noticed something curious the other day. The flow appears to me to be over and the girls are getting protective. I watched the guard bees repell what can only be described as robbers (about one every 10-15min). The robbers would dart back and forth much like orientation flight except a little farther from the landing board and on a mission. As soon as they approached the landing board (seemed to aim for the side of the entrance) the guard bees would rise up in unison and repel them. One guard bee takes her job so serious that she will not let ANY bee in her little 2 square inch of landing board. No bee takes off, lands or walks through her domain...kind of humorous and entertaining to watch.