This happened with my grandmother, my mum hasn't spoken about it much but from what I know of things she has said as well as bits from my aunts/uncles this is what happened;
GM married young, had a baby, then found out she was pregnant again when her firstborn was a few months old, she was so distraught she tried several things to try and miscarry (which she also did with each pregnancy). It didn't work and she had more children in quick succession, she ended up with 5 children under the age of 8. GF decided after the 5th baby he didn't want to be a husband/father anymore and just left.GM went straight into another relationship and had 2 more kids (less than 2 years apart), I don't know how soon after the youngest was born that the boyfriend left but he was never heard from again.
When the youngest was a few months old GM left the oldest of the 7 children in charge whilst she popped to the shop for a loaf of bread, she didn't come back. Great grandmother looked after them for a number of years, the 2 oldest children were the most affected by being abandoned not only by their dad but also by their mum, they ended up in care because great GM couldn't cope. When GM eventually got back in touch she had moved quite far away and wanted her kids back, so the youngest 5 went to live with her, the second oldest eventually moved to the area (when she was a young adult). The oldest child never forgave GM and they didn't speak, when GM was dying my mum and her siblings reached out but oldest sibling didn't want anything to do with any of them. The second oldest had a very strained relationship with GM but my mum and the rest seemed really close to her. When they talk about her there is nothing but love for her so I think they forgave her completely for what happened.
I didn't find out about any of this until after she died, it seemed so bizarre to me as my GM was a really caring GM who would do anything for her grandkids and great grandkids and was very hands on(I have many happy memories of sleepovers at her house and holidays/days out with her). I think she had some sort of breakdown, sorted herself out and then tried to make up for what she had done, unfortunately, the damage was done with the oldest 2 but particularly sad that the oldest sibling never spoke to her again and doesn't speak to any of their siblings (I'm sure there is probably more to it but I'll never know).