Check out this break after 2 minutes in the microwave:

I’d nuked it with the lid on, but askew, so that the sauce wouldn’t splash, but heat could still get out. Look at how close to the sauce line the break occurred:

I was actually able to take off the top half of the container, and nothing spilled!  It was such a clean break that I probably could have eaten the sauce in the center of the container (I didn’t, and threw it all out to be safe).

I’d guess there was a hairline fracture somewhere around the “sauce line” and the heat differential with this particular loading was enough to stress that defect enough to break all the way.