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.

As MCheng said that is very likely botulism. You will need to watch for symptoms for 10 days. If you develop symptoms then you need to get medical attention immediately. It doesn't take very much to be in danger so unfortunately even the amount you ate can pose a risk.

From the CDC's website:

If you develop signs or symptoms of botulism, seek medical attention immediately. These include new onset of double vision, blurred vision, drooping eyelids, slurred speech, difficulty swallowing, or muscle weakness. If untreated, the illness may progress from head to toe, with paralysis of the face, arms, breathing muscles, trunk, and legs. Symptoms generally begin 18 to 36 hours after eating a contaminated food, but they can occur as early as 6 hours or as late as 10 days. If your physician suspects botulism, he or she should contact local or state health officials, who may collect any leftover product and cans.

If you ate a recalled product in the past 10 days, no intervention is available to prevent botulism. Treatment is only given to persons who show signs or symptoms of botulism. If you develop signs or symptoms of botulism, seek medical attention immediately. You do not need to make yourself vomit or take any other action. Watch for signs or symptoms of botulism for 10 days after you ate the recalled product.

If you ate a recalled product more than 10 days ago and you have no signs or symptoms of botulism, you are very unlikely to develop botulism.

Also, please make sure you dispose of the contaminated food correctly:

Do not open or puncture any unopened can of the recalled product. Dispose of food that may be contaminated by placing in a sealable bag, wrapping another plastic bag around the sealable bag, and then taping tightly. Place bags in a trash receptacle for non-recyclable trash outside the home and out of reach of humans and pets. Do not discard the food in a sink, garbage disposal, or toilet. Avoid splashing and contact with the skin. Wear rubber or latex gloves when handling open containers of food that you think may be contaminated. Wash hands with soap and running water for at least 2 minutes after handling food or containers that may be contaminated. Wipe up spills using a bleach solution (use ¼ cup bleach for each 2 cups of water). Completely cover the spill with the bleach solution. Place a layer of paper towels, 5 to 10 towels thick, on top of the bleach. Let the towels sit for at least 15 minutes, then put the paper towels in the trash. Wipe up any remaining liquid with new paper towels. Clean the area with liquid soap and water to remove the bleach. Wash hands with soap and running water for at least 2 minutes. Sponges, cloths, rags and gloves that may have come into contact with contaminated food or containers should be discarded with the food.