Whether it's a raid or not does not affect the chance of prisoners being taken. Pagans and Zoroastrians double the chance of taking a prisoner if the character is a woman.
Anyone who is at least rank 2 in a warrior lodge multiplies their chance by 10 (!), as does anyone who has a reaver bloodline (the ones who give you an extra chance of looting artifacts or taking prisoners).
All these stack, so if you have a warrior lodge member of rank 2+ with a reaver bloodline we get 100*10*10 = 10,000 weight of taking a character prisoner vs. 1,000 weight of not taking them prisoner. Make that 20,000 if the character is female.
This way raiders and attackers can quite easily take most or all potential targets prisoner.
Yeah, that'd explain it. Even a mere rank 2 pagan dude with no other modifiers moves to 2000 weight of capturing females vs 1000 of not doing so. 66% capture will see "everyone" captured as a not too uncommon event, especially if the actual numbers are small. And most of the eligible targets are preselected as female anyway, since it's women and child relatives, and rulers at the location that are checked for.