I think a huge thing is the self worth of women.
Either some women don't have any or very little self worth that they end up in relationships with men who can be abusive - sexually, physically, emotionally, financially and these men either leave or have little input into their children's lives or the women in some cases through support of other people and organisation's find the strength to leave.
You then of course have young girls who perhaps had unhappy childhoods and all they want is a loving family so possibly have children at different life stages than their peers who had more stable and caring upbringing
There are some people that didn't actually plan and discuss expectations of family life and after possible years of misery and sometimes ending up with a partner who is like another child they go their separate ways
One night stands / poor sexual health planning
Widow/loss of partner
Partner in prison
Women's preference e.g. IVF baby via donor
Also a thing I think is the breakdown of the extended family - over the years it has become more accessible for people to attend university which for alot of people is away from where they grew up, this then opens doors to live and work in areas away from parents, meeting and forming families with people whose parents are in yet another area of the country
For example neither myself or my brother live near our parents or each other. We both have young families being raised away from 1 or 2 sets of grandparents, grandparents (can) play a key role in childcare thus reducing pressures on parents so for those families with no wider family help breakdown of relationship may be more likely.
I don't actually know any single parent families in my life currently but I assume in 10yrs time I will not be able to say the same thing