Breakage at the head is not unheard of...
It's caused by excessive stress induced prematurely during the mushroom process - i.e., that stage at which the end of the spoke is hammered to create the mushroom head. Maybe the spoke didn't get the rigid support it needed while receiving a direct line non-deviating blow.
Usually it'll break before or while on the way into the bag/box - i.e., headless spokes will be in the bag or box with the little heads floating around.
Undertensioning of the spokes in a complete wheel will quickly finish off the rest that have already been fatiqued at that poiint.
Cold forming is usually a pretty reliable process - but then again even that process is at the mercy of the humans applying it and monitoring it for QA purposes.
=8-)
__________________
5000+ wheels built since 1984...
Disclaimer:
1. I do not claim to be an expert in bicycle mechanics despite my experience.
2. I like anyone will comment in other areas.
3. I do not own the preexisting concepts of DISH and ERD.
4. I will provide information as I always have to others that I believe will help them protect themselves from unscrupulous mechanics.
5. My all time favorite book is:
Kahane, Howard. Logic and Contemporary Rhetoric: The Use of Reason in Everyday Life