What causes rotten smelling diarrea?

Undigested food: If the bowel transit time is so rapid that food does not have time to be digested and absorbed before it gets to the colon, it will actually rot in the large bowel.

Lots of things...: Ranging from what you have eaten, e.g. Onions, garlic, spices, excess (mega dose)vitamins, inflammation, infections with viruses, certain bacteria, meds, blood (usually gives black or maroon stool), poorly absorbed fat or carbohydrates, other.