Broken bicycle rim

Even with all 111,000+ miles I have ridden on my bicycle over the years, a new first for me.  The rim on my rear bicycle wheel broke.  Even with all the weight I carried on bicycle tours years ago on other wheels, those wheels held up.  I have no idea why this rim broke now.  I don't remember hitting any major potholes or other rim damaging things.

I went out to my bicycle boneyard this afternoon and found another rear wheel.  The tire was flat.  I filled it with air and so far the tire is holding the air.     I'll see tomorrow how the tire and rim hold up.