They occasionally don't. It's an issue with the LOD mesh for dead military (non-Zed). It's being fixed in the next patch, though, which shouldn't be too long, as it's just a hotfix. For now, just pray you don't get shot when it's messing you up.
The best way to fix it seems to be anything that restarts the rendering engine. Also, keeping your videocard cooled properly seems to minimize how often it happens. Having VRAM set to default seems to help, as ARMA 2 doesn't seem to do well allocating more VRAM than your card actually has.