Originally Posted by FBinNY

Let me guess, you have a narrow rim. If I'm wrong don't bother reading on.

When tires are mounted the thickness of the two bead areas inside the rim leave a very narrow gap between them. When the tube is slightly inflated it fills the tire and lays across that gap. As the pressure increases the strip across the gap blows down through it. In some cases this hyper-stretched section splits like an over-inflated balloon.

Your case is slightly different, in that the problem is limited to that nearest the valve. What's happening here is evidenced by that small over stretched area just at the edge of the reinforcement. The base of the valve is above (tire side) of the narrows I described, and the base reinforced area is too thick and stiff to blow down, so it sits up there leaving a small gap below. The first unreinforced section blows down and then across trying to fill that gap, and simply can't stretch that far.

The remedy is extra care when installing. Start by using talc generously inside the tire so rubber can slip on rubber. After installing, but before inflating, push the valve down between the beads until you feel it bottom into the rim. If you don't trust your fingers, put the valve through the rim before installing the tire and note how much sticks out). Then inflate to 5-10psi and bounce the wheel around a bit to try to let the tube equalize. Also do the normal seating check, then inflate to riding pressure.

Warning--Do not pull the valve out by the stem, this can tear it from the tube --


Edit -- AS wrote a shorter explanation or typed faster and got his in while I was still typing.