For some time I had noticed and been avoiding the weird bulge in my rear bicycle tire. Finally it gave way on my commute. The threads inside the tire appear to have been separating, and I guess I hit something sharp at a weak spot.
I put on my spare tube and made it home. I have since replaced this tire with a knobby road tire, paired with a puncture-resistant inner tube and a puncture-resistant liner. If I never have to change a bicycle tire again it will be too soon!