One. The drop

Through pregnancy, oestrogen and progesterone run at levels the body never sees otherwise. Within days of delivery they fall, steeply.

For most women the nervous system rides that out. For some it does not, and the current understanding is that those women are unusually sensitive to the change rather than deficient in anything.

The reason to trust this account rather than treat it as a guess is that medicines built directly on it work. Drugs that act on the pathway that the drop disturbs lift mood in this specific population, and they do it in days. That is suggestive rather than settled, and it is the reason clinicians treat this period as having a biology of its own.