You can read this article for all the details.
I am not sure what motivated them. I know it stated that they all wanted to raise their babies together, but honestly they have no clue what any of that includes. Raising children is not easy, even when you are married to a wonderful husband. Are they looking for someone to love? Do they need to feel a part of a family? What message are they getting from their friends, family... anyone?
This is one reason why it is so important to teach our children the way they should live. We are to teach them to love the Lord and obey His commandments. (Deuteronomy 6:4-9)
We as parents are also to love our children. I think that many teens go looking for love because they have not received it from their families.
Here is a new update to the original story!