I saw this the last time out, and it more-or-less corresponded to the momentary changes in transparency.
I think this can also be small scale, too, as I see it often when viewing a large low-surface-brightness object. The details in the galaxy or nebula seem to come and go over a span of minutes.
There is not any noticeable cloudiness when this happens (blue skies at sunset and dawn and no high cirrus passing through)