I have had this issue occur for two reasons that I have discovered (so far).
The condensate is the first and most common reason. Doesn't happen a lot but it happens enough that I've gotten used to it.
The other reason is the sun's angle. Depending on the season and what time of day I am driving and the direction, sometimes the sun is low enough in the sky, and the camera has a clear view of the sun, the camera will get "blinded by the light"... hmmm that phrase might make a good song...