I've read a number of responses in this thread saying if it's more than one child estranged then it's probably the parents' fault - but if it's only one child estranged it's probably that child's fault.
Regarding the latter opinion, I'd like those responders to consider the following:
In many dysfunctional families what often happens is that all the issues that the family won't face are dumped by the parents onto one child, who then often unwittingly fulfils the role of the designated scapegoat.
Over the years this child becomes the repository of the family's problems, the 'problem child,' because they are often the canary in the coal mine who speaks up about things that aren't normal.
And for having the audacity to speak up and question the insane status quo, this child is squashed, punished, gaslit and becomes convinced that they are in fact the cause of all the problems.
Over the years, this dysfunctional coping strategy on the part of the parents means that they can avoid facing their own massive issues and can continue to convince themselves that they are just fine and can also look wonderful to the outside world.
A mentally ill mother, for example, and an enabling father can present themselves as wholly innocent victims and perform this act of saintly faux-innocence ('I just don't know what we've done wrong') which is often extremely convincing to all and sundry, as most people want to believe that most parents are decent people. Meanwhile the scapegoat child is alone, demonised, sad, disbelieved, minimised and misunderstood, usually with no help forthcoming.
As one of a family of five, I can categorically tell you that all my siblings and I found our parents' behaviours extremely challenging. However, I as the scapegoat was the only one who went NC for a time as a middle-aged adult. Basically, I had to protect my own mental health as I had some significant complex PTSD symptoms.
Predictably my parents 'had no idea' why, even though I told them. They refused to listen when I tried to explain so I then wrote a very clear and detailed letter to them explaining the decades long patterns of emotional abuse which culminated in their ordering me out of their house when I alone faced them and represented myself and my siblings about a range of the issues.
So yes, I was the only child in my family who became estranged from my parents. And many people - and apparently many here - would automatically think it was my fault because on the surface of it I was the only child to do so in my family. People are of course welcome to their opinions, even when they have no idea of what they're talking about. However, my decision was in reaction to decades of dysfunction, a decision to save my sanity. I was brave enough to do it. But it's not for everyone. It was extremely difficult and I felt an enormous amount of pain over it.
I have also worked with many people who are estranged from family members and I can tell you that no child ever takes this decision lightly. It is often only done after years of soul-searching and after trying absolutely everything, including trying to be understanding of parents who continue to prioritise how they look to the outside world over taking accountability for the highly problematic ways in which they related to their children.
I forgave my parents long ago. My father died last year and we had some good moments together before he passed. My mother is still who she is, a highly challenging individual who can never see herself as anything less than 'always wanting the best for everyone.' I visit her weekly in her aged care facility for an hour or so because she's lonely. It's what I can live with.
tldr: I think people who make snap judgements about who was at fault are either lacking in experience of estrangement or they didn't have difficult parents. And to those people I say: how lucky you are.