Scrub , I have worked around some dump equipment . No hydraulic lock valves because the hoists are powered up and gravity down . Some of the old hoists had a limit cable to stop the bed from over extending the cylinder , If you bumped that cable in the least little bit it would drop like a rock .
In my opinion years ago dump beds should have been manufactured so the prop or blocks were kept with the truck .
Older trucks used linkage to operate the valves and bumping the linkage would bring the same results . Very dangerous deal, but most people never give it a second thought .