For the most part, I think it is a slow burn before getting to 'having more DC with an arsehole'
When you first date, you are living separately, getting on with (and maybe enjoying) work, career, friends, social life. You start by spending your leisure time together without any of the 'daily grind' interfering. Then you spend more time together and look to the future, holidays, maybe moving in together, marriage on the horizon - still the focus is off of the daily grind. You are both still independent and likely have your own income and independent social life.
As you move on and perhaps move in together you might pool some finances and have niggles about each others living habits - but that's natural, right? You might whinge a bit but you still have plenty of time to deal with it, can go out for meals on a whim etc
Then you decide to get married and have children - very exciting again.
Once you get pregnant it changes. You are more inextricably linked. Women very often lose their 'independence' and social support during ML, moving away to follow DH's career, other friends caught up in their whirlwind of juggling homes, children etc. You have your family 'unit' which society holds up to be all you need and many women lose their sense of self.
At this point (pregnant or newborn) women are at the highest risk of DV
. Men who were the centre of a woman's attention are not now (and probably quite rightly for a period after the child is born). And rather than helping and supporting DW through it they retreat and say 'well I'm earning so it's your job to deal with it'. As another poster said, the women step up to the challenge and get on with it. The downward spiral continues. There is no time to maintain the full independent social life/support and people become more insular within their 'unit'
I know this isn't the case for all relationships but I think in many cases it is a such a slow almost imperceptible decline from 2 independent adults to an abusive mid/post DC situation. When the women emerge from the 'parenting fog' they can step back from it and say WTF???? But in many cases it is too hard, so they accept it and it continues 
^massive generalisations there I know!