I was in the Nether and I placed a torch on top of some gravel and all the gravel around me fell out from underneath me. It was probably a group of floating gravel that spawned, but is it supposed to fall if a torch is placed on it? I placed the torch on the side of the block facing West if any of that makes any difference. I'm sorry I didn't get any picture evidence and I'm hesitant to attempt to do this again because I think floating gravel is kind of uncommon and I don't want to die again.
EDIT: I tested this on creative mode. Any block will cause this to happen.
To recreate:
Find floating gravel.
Place anything onto the floating gravel or above it via the wall. (It just has to be in the space above the gravel.
The gravel and all surrounding gravel will fall.
This is completely works as intended. Floating gravel is suppose to fall upon any block update (as its a falling block). That includes placing a torch on it.