Similar disasters have happened in badly designed sports stadiums and at music festivals. Too many people in a small area, crowds pushing from behind and people getting crushed.
Only a couple hours earlier, I watched a live-stream from a outdoor street Halloween celebration in Tokyo (because I'm a bit of a japanophile, sosumi...) There, there were cops with megaphones doing crowd-control to keep people moving, preventing them from crowding too much in one area. I found them really annoying then, but after Seoul, I see them in a new light.