It is those darn U.S. Customs and Border Patrol agents.
The Canadian guys are nice, but the U.S. fellas are always looking for contraband. They probably removed the window and snuck in to take a peek inside the trailer to see what you might be smuggling across the border. Contraband liquor, Bulgarian prostitutes, cheaper pharmaceutical prescriptions, etc. When they found out you were clean, they probably just kept the window out of spite.