If there's gravel (and I suspect this also works for sand, anvils and concrete powder) on top of you and you have nowhere to go, instead of suffocating you, it pushes you downwards. If there are no air pockets beneath you, you fall through the world. Easy to reproduce: go into superflat, make a little 1x1 room and put a gravel block on top of you so it falls on your head while in survival mode.