You shall no doubt have observed a perfectly dry dog shake imaginary water off itself. This seems to be a tension release mechanism that marks the end of a period of increased alertness in the face of potential danger. I have once read someone referring to this as ‘going back to neutral’. So if you see your dog do this although he is perfectly dry, it is likely he has just been subject to some tension that is now over.
With clever timing, you can teach your dog to shake on command, thus giving you more control in potentially hairy situations.