Autolysis is something people fear but rarely actually happens in beer. Mirmite, burnt rubber, and soy sauce are all flavors associated with autolysis, so it may have happened to you.
What happens is the yeast aren't able to build thick, strong cell walls. So when they die, instead of just settling to the bottom of the fermentor, they rupture.
Insufficient oxygen, insufficient nutrients, and temperature extremes can all contribute to it.