I used to work in the fastener industry, and there could be a number of reasons why this happens: hydrogen embrittlement
during the zinc electroplating process, burst heads during the heading process, heat treating not to spec, to name a few. It can affect an entire production runof screws; we used to make them at a few hundred thousand at a time, and this wasn't common, but it wasn't unheard of, either. But it is rare that it would get past the QA procedures. Screws made "offshore" aren't as good as ones made here, unfortunately, so that may be part of the problem. Whenever I encountered broken heads, I just remove what's left, replace it with known good screws, and move on.