When I took my bike off the hook in the garage today I found the rear had exploded. Everything was fine when I rode last Saturday but today the tire had a 4 inch jagged gash in the center of the tread. The tires are Specialized Pro Roubaix. The sidewall reads 120 psi and I had it inflated to about 125. I guess that answers my question about why it blew but it seems odd that it would happen while it's hanging the garage. Glad it didn't happen while I was speeding downhill.
I thought the tires could handle a little over inflation but I'm going to be a lot more careful from now on.