Sounds like a failure of the "web" joining both sides. It fails at the valve because that's where the hole to h ole distance is very short.
You didn't say, but I'll venture that you're inflating to fairly high pressures. That's important because the "hoop stress" in the tire, or the force pulling the rim flanges is equal to the tire pressure X the tire cross section.
Add to that the greater amount of load deflections as each spot passes the bottom, and you have a possible explanation for early failure. Another consideration might be excess spoke tension, but that usually manifests in cracks at the inside (toward the hub) surface rather than the web.
BTW if you have rim brakes that's also a contributing factor
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