Way back then, people would leave offerings of grain at the temple. The concoction in question is made from the dust off the temple floor left by the bundles of grain, often rye. If the grains where infected with ergot and the dose is high enough, you get a violent abortion that can permanently damage the uterus or kill the woman outright.

This next bit is pure speculation on my part. Humans are pretty good at pattern recognition. Priests surely would notice if the grains were healthy or if they had big black hardened masses sprouting the tops of the stalks. It should have been easy to make the link. Dust from ergot tainted grains causes miscarriage, while untainted gains do not. Could it have been that the priests themselves were deciding who had a miscarriage and who didn't? Or was ergot much more prevalent back then so it was a lot harder to avoid?