A specific technical reason:

A specific example of such a case was been observed in the Canopo peoples of the Dougon taiga in Northern Gachaza. A mutated species of parasitic insect called the flockfly aggressively attacks the mammary glands of larger mammals.

For some reason this fly is particularly attracted by the odor of secretions from Montgomery's glands in human women.

Flockfly infestation is relatively rare, but when it happens it is devastating to livestock, not only due to the damage to milk production but also due to secondary infections.

The infestation is difficult to eradicate once it has taken root in a herd, so to reduce the chances of flies being brought in from outside, women of childbearing age and older are prohibited from traveling outside their own communities except in direst circumstances.

Flockflies are generally only attracted to women who are pregnant or lactating, but the taboo on women traveling as a result of this phenomenon has become generalized to all women.

A specific technical reason:

A specific example of such a case has been observed in the Canopo peoples of the Dougon taiga in Northern Gachaza. A mutated species of parasitic insect called the flockfly aggressively attacks the mammary glands of larger mammals.

For some reason this fly is particularly attracted by the odor of secretions from Montgomery's glands in human women.

Flockfly infestation is relatively rare, but when it happens it is devastating to livestock, not only due to the damage to milk production but also due to secondary infections.

The infestation is difficult to eradicate once it has taken root in a herd, so to reduce the chances of flies being brought in from outside, women of childbearing age and older are prohibited from traveling outside their own communities except in direst circumstances.

Flockflies are generally only attracted to women who are pregnant or lactating, but the taboo on women traveling as a result of this phenomenon has become generalized to all women.