Crashing and then deleting the level.dat_old file

I was just playing in my world when suddenly Minecraft crashes. I used alot of time to figure out what the hell happened. I found out that my level.dat_old file was missing! I fixed this by making a new world and took the file from that world, by doing that I did lose my enderchest ''stuff'' and my inventory.
I have no idea why the crash appeared, it just happened randomly.. :/