Hand luggage
The fact is that pilots and cabin crew train for these sorts of situations. SLF do not. I know there's a safety briefing and they have a safety card to read but that is not the same as the sort of training in evacuation procedures that the pros get.
Point being: the overwhelming majority of SLF will never experience anything like this. Virtually no one will go through it twice (and if I find myself sitting beside a PAX who has, then I want off that plane before he has a third emergency situation).
So - when faced with extreme and unusual high-stress situations, people react in extreme and unusual high-stress ways. I'm no psychiatrist but I bet that what's going on is a sub-conscious process of trying to grasp onto something that is normal: "I'm getting off the plane. I've done this before. I must get my hand luggage..."
Moreover, they probably reckon that, as they have to stand and wait their turn to get the hell out, they might as well just grab their bag. Admittedly, they aren't thinking that it might impede their progress or tear the slide - but then, these people aren't professionally trained at getting the hell out of an airplane quickly.
I'm sure there's a reasonable number of idiots on these planes, too - but I just think that sometimes SLF needs to be cut a little bit of slack.