Wow, this wasn't a great post to read, just on the fact that I am SO paranoid about this happening EVERY time I take my buddies subaru. I am always looking in the rear-view mirror every 5 seconds making sure everything is ok.
Maybe it happened to loosen over time with highway vibration? I usually shake and check if everything is locked down before driving away though, again - my paranoia.
What exactly malfunctioned? Did the entire rack just break off? What brand was it and how old was the rack?