Gravity!!!
Not sure why they fall, but I bet ALI and OSHA have statistics on why these events happen (or at least the reported ones)
Some of the YouTube videos are interesting as well as an eye opener.
I had my 21' Grady White fall off the blocks while I was bottom painting it years ago so I'm extra parinoid about heavy stuff falling on me. Luckily I heard the board that was under the bow start to crack and rolled out from under the boat as it fell.
This is one of my worst fears, so I go way over the top when lifting a vehicle. I follow the ALI lift guide lift points along with the lift point recommendations in the factory service manual. I literall get on my knees to line up the pads with a plumb, then I elevate the car a few inches off the ground and recheck to confirm the pads are where I wanted them, then I shake the vehicle to make sure it is secure. Then I raise the vehicle and set it on the safety locks. Then I visually inspect every pad location and visually ensure the safety locks are fully engaged before walking under the vehicle. Since both sides of a Mohawk can be operated independently I also make sure both sides are on the same safety lock (I numbered the holes). I only had them out of sync once which was the first time I lifted my jeep, but it creeped me out enough that I check the number next to the lock on both sides every time. Even after all this, I still keep an eye on the vehicle while working on it.
This sounds like a lot, but it really only takes a few extra minutes. I have a hobby shop so time isn't important to me. Being safe is.
Chris