Water vapor hitting a poorly insulated cold area then turning into liquid water maybe. 

Because the substance is cooked, its vapors can also cause accumulation in the ducts and air handler.

.- Freon leaks on units less then 5 years old- galvanized pipes in attics with pinhole leaks- cracked slabs (and all that goes with that repair from cracked tiles to cracked walls)- AC condensing unit stolen in vacant house- underground plumbing leaks- and the best one was a voltage surge in a house that burned out the transformer on the AC unit, and burned out the stove digital display, and ruined two outdoor mercury vapor lights.

Less Maintenance in changing bulbsNow if they only came out with LED Mercury Vapor Bulbs!!!!!!!!

The good thing is that there is a vapor barrier on the underside of the upper floor which is the double wide itself.  

The inspector noted some standing water in spots and mud in the crawl space on top of the existing vapor barrier.  

tenant with asthma saying that even the vaporizer from other students giving him asthma.

Get a nice vapor barrier, or better yet, seal the concrete before you do the install.

Does the crawl space have a vapor barrier.

This is why we don't put a vapor barrier under laminate laid on plywood.