if your defrost is on your truck will run the compressor to dry the air, problem is when you shut the truck off the moisture hangs on the a/c condeser in the HVAC, and then when you start the truck it can sometimes force it out and it will collect on the window if its at just the right tempurature. my superduty does it all the time. only way to prevent it is to turn off the defrost for a few minutes before your turn the truck off, or wait to crank the defrost once going again. 
