Reciprocal obligation is the key. I've heard people spin it so that children owe their parents an infinite debt that justifies anything, but that is not reciprocity. Reciprocity means equivalent behavior on both sides. People get a chance as adults to discuss what happened in their households as children, and some people find out that their stories are radically outside the norm. Usually all they want is some acknowledgment from their parents that mistakes were made and it wasn't ideal. Denial and justification are the things that trigger estrangement. Or continued exploitation: people who cared for their drug-addicted or otherwise needy parents for years starting in childhood and decide that however much their parents still need them, they have to separate themselves from that pathology so they can have their own healthy life.

Not everybody wants to share details, but I've heard enough stories from friends that I'm shocked what people are willing to forgive from their parents. I shared one story in another comment -- that person has a close relationship with their parents despite it not being a very positive one. Another friend of mine, who is queer, grew up with a father who would frequently say things like all gay people should be executed, they should be locked away from decent people, doctors should have let AIDs finish the job, etc. This person did cut off contact with their parents a couple of times but eventually heard sincere apologies and regret from their father and reconciled. That was almost twenty years ago, and now they speak lovingly of their parents and are helping care for them as their health declines.

Really it comes down to two questions: forgiveness and continuing harm. In my observation, adult children are not stingy with forgiveness for their parents. Not everybody can forgive everything, and it can take a while (I wouldn't bother asking until the kids are at least 25, maybe 30) but people forgive their parents for things they would never forgive anybody else for. They also tolerate a lot of continuing harm for the sake of maintaining the relationship. They draw a sharp line when it starts to affect their own children, directly or indirectly, and I think it's good for everyone that they do so.