One possibility would be a awareness. Like in a modern-day society, medieval brutes finally realise that women are human too, liberte, egalite, fratanite, hundred years later: voila, women in the army. I'd like to highlight another one.
No-one else is left
There is an old grim saying. "When are 16-year-old drafted to army? When there are no 17-year-old left anymore."
Female warriors in larger quantities would arise if almost all males (the usual army material) are already killed off. This does not need to be global or even permanent.
To give an example, if all men leave from a castle (unwise, I know), gentry ladies might take weapons into their hands. (A few ifs: it should be Ok to do so, and not only play a damsel in distress, as there are men left, but of lower caste.)
To give a more striking example: Imagine a village in 30 years' war. All men are in army of the landlord. Marodeurs rampage around. Human life is very cheap. Women would bundle-up into some kind of an all-female squad, even armed with improvised weapons, in order to protect themselves.
A corollary is that next generations get a gap. The so-called "demographical echo of the war" would be even more severe in the purposed case. So either some fantasy-level uterine replicators come to the rescue, or the demography drops. The historical remedy from the 30 years' war was to temporarily allow polygamy. It might even not help.
But lo and behold! The effect can be local. So, the one side looses the war, even despite women at war. There should be winners, the now-conquered province struggles to increase its population.
Should such an effort be indeed global (and in absence of artificial childbirth cycle), it spells bad for human race, similarly as an all-in war where almost all males are killed.
There was an historic 20th century effort, but not that large-scale, and it's definitely not medieval.