So unless blocks can kill you this shouldn't happen. I think it's lag, where an opponent switches his weapon and the game forgets to tell you, like when people teleport. This also happens when you change weapons with ".", not just the mouse wheel.

Can it be changed so the game checks which gun your opponent has every time you die? It would fix the "." weapon switch lag so people wont think they were sniped with shotgun when the player was really holding a rifle.

It happens if you accidently fall off of a cliff or something.

nah he crushed your head with a block :p