Which brings me to the point in posting this; Cave ins would add a lot of depth to mining. We already have tiny cave ins with gravel, but those only cover one block. Implementing this as a full blown feature would make mining a lot more dynamic. A certain material that can be uncovered up to 2 blocks, but after 3 has potential to cave in. This would force the player to be more strategic in building their tunnels, and place support beams or frames like you typically see in actual mines.
It's not a glitch. Sometimes chunks are generated with gravel/sand kind of floating in the air. When you remove any of the blocks near the gravel/sand, all the gravel/sand nearby will then fall.
Something like this has happened to me a few times, and one time, the cave was actually an ocean. Imagine my surprise when digging out some coal when the nearby sand roof entirely collapsed and dropped a bunch of water on my head.
EDIT: oh, you already mentioned cave-ins, heh. But yeah, having more caves with sand/gravel roofs would definitely be pretty cool and more interesting.