I am guessing that it was overloaded. A 5x10 is probably rated for three adults thus designed to load to 1000 pounds, but if you have a keg plus ice at 250 pounds and you get four or five people maybe grooving to music it is easy to put more that 1000 lbs of force on it. Dont know what was on it at collapse, but the above scene could happen on that space.
Not too many years ago I was taking a kid to college, we drove the elantra because the pass was a mess and that Elantra does a lot better in snow than my Chevy conversion van did. College kid had supplies along too, loaded out the back. I was concerned that we kept bottoming out, thought that I had suspension or spring problems. My mechanic calmly pointed out that the car is only rated at about 900 lbs, that it was 8 years old, probably could not do it anymore, and we just maxed it out. THe car is rated for 5 passengers and we only had 4, and the luggage fit, so all should be good right? Not.