Ignorance of birth control for one thing. Momentary lust and lack of self control for another.

Having worked with teenage mothers, I think that many, if not most, want a baby to love and to be loved. Unfortunately, reality soon kicks in!

Also, if girls have have low self esteem, no qualifications and poor job prospects,  being a mother can make them feel special, partnered or not.
Again, reality soon kicks in!

A teen girl that does not have a caring loving father at home will seek for that male attention elsewhere, and there's plenty of walking testosterone out there willing to oblige. It always helps when a family has standards set not allowing their teens to be alone with the opposite sex, no matter how much you trust your kids.