Why did my storebought spaghetti sauce bubble out of the jar? Tonight we were having meatball subs we open store bought spaghetti sauce and put it on our subs. We just spooned it out of the jar, no heating. We looked away for a moment, then once we started eating I looked at the sauce and it had bubbled and run over the sides of the jar. The top looked like lava with bubbles busting air. Anyone ever hear of this? What happened? It was sauce with Italian sausage.
Since it was unheated, this is definitely a sign of food contamination. Since this had Italian sausage, it is low pH, and could be in danger of botulism toxin. Here is information from the CDC about botulism. If you ate the sauce, I recommend you see a doctor quickly, as it can lead to paralysis. 

If it had been heated, it could have just been boiling over. As liquids boil, bubbles start to form. Since the spaghetti sauce is thick, it's going to trap the bubbles, and release them with more force. The bubbles may accumulate so much, that it will spill over the pan.