Originally Posted by
jwible
Pump up the tube to about 5 psi, check all around the wheel to make sure the bead is seated, and the tire isn't pinched. Finally, jam the valve stem back into the tire to make sure the tire bead at the stem gets below the tube. That's the area where the bead is most likely not to seat well. Pushing up the valve stem lets the bead get around the tube and seat properly.
This is a likely cause. If you don't push the valve down into the tire, the edge of the tube can be caught under the bead right at the valve. Then it eventually pushes the bead off, blows up like a balloon, and Bang!