Mine do head bob when excited, especially around the pool. It's nothing quite like this though.
How have they been acting aside from this in terms of eating/drinking, pooping, and general behavior? Have they been doing it on and off, or is it all day? Do you notice it more during specific times?
My first thought was that maybe some food could have gotten stuck. Mine do something sort of similar when they try to eat dry food too fast. The problem has gone away since soaking their food. I don't know if this is anything related as it looks a bit different and it is on-going though. I hope somebody else has some ideas.