Huh?

So I was being domestic and cooking supper. Had some fish to fry and some rice and Terri got out some broccoli. I had put the fish on the back burner so as not to spill anything on the floor. Its frying real good and I am getting hungry smelling the aroma. The broccoli is on the front burner on the same side. I raised the lid to see how it was doing and got a whiff of the worst smell I have ever experienced in the kitchen. Why does broccoli cooking smell like a rancid fart? I like eating broccoli, especially with lots of butter. But I don't guess I had ever smelled it cooking. Not sure how that will impact my desire for broccoli in the future. The fish is really good but I must find something else to eat with it.