You trapped the tube under the tire when you mounted it. It eventually lifted the tire and blew out between the tire and rim.

When you install a tire and tube, the tube must be inflated until it is round, throughout the mounting process. After the tire is on the rim, go around on both sides, pushing the tire bead inward and looking to make certain that you can't see the tube anywhere. Then pump up the tire. After pumping, look at the tire on both sides and see that it seems evenly mounted.