Assuming that it's an older wheelset it's possible that it largely sat unused but I'd think unlikely. So I'm shooting for undertensioned at some point that weakened a lot of the spokes. That and maybe something in the hub design that put a higher degree of loading on the head than normal.

Spokes with defects in manufacturing should/would show up their problems early and not after many years and I's assume more than a few miles of use.

But since we haven't heard about the wheel's history all this is just conjecture.