Why did my storebought spaghetti sauce bubble out of the jar?
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.