To understand why some women have children within a a short time with multiple different partners?

Again, other people’s choices. The OP doesn’t get a say in how other people behave, whether that’s recklessly reproducing with a string of unsuitable men, or being the grandparents picking up the slack over and over. If she’s likely to produce several more babies, if she had three and is only 25, at some point her parents will run out of capacity, and some of her children will end up in the care system, which may act as a wake-up call. Or not, obviously.

Apparently lots of short term, sequential sexual relationships has been labeled by those who study such behaviour.

I would give her a wide berth, if I were you OP.

But the grandparents are picking up the slack, so "her body her choice" isn't a valid argument.

Her body her choice?

I mean, to answer your question, there could be reasons ranging from purely being totally feckless and having a touching faith in other people's ability to pick up your responsibility, to having some tragedy in younger life and horrible loss that you tried to fill with having babies to experience the love otherwise missing or lost. I think you probably all need her to just stop though, whatever the reason is.

Did you mean to NOT understand in your title?

I don't understand it because for me it would be an absolute ball ache. I couldn't cope with having to manage 3 baby daddies.

I do think about what some people have endured, what they've grown up in, what they've learnt in that environment and therefore what template has been set in their mind. I'm not for parent blaming, but this sort of approach comes from somewhere. A desperate hope that this one will give her the little family dynamic she always wanted maybe? But the reality isn't working that way because she's picking or drawn to unavailable men and does not even realise it.

I don't know. I think things are often more complex and multi layered than we like to think.

I don’t understand these kinds of life choices. Nothing is gained aside from increased impoverishment. It certainly does not benefit the children. She isn’t thinking logically so reasoning won’t work. Best just to be there for her emotionally when needed (and within reason).