I have noticed this at times as well. I always assumed it was small organisms moving in the water column during different light conditions. Anybody have any thoughts on this? I've never paid enough attention to it (I don't night fish much) but if this is what it is it will likely be more intense during a new moon and less intense during a full moon. See if it correlates over your next couple night trips. Also found this after a quick Google search:
https://en.m.wikipedia.org/wiki/Diel_vertical_migration