I'm not sure why you posted this here, and I'm worried you won't see my reply because this is the only way I can respond to it, given you posted anonymously. But I'll give it a shot.
Why social services chooses to investigate families is sometimes very arbitrary, but also sometimes about more than is just on the surface. For instance, I have no idea about the context this happened in per her partner (if it was consensual, setting aside that her age alone may make it nonconsensual). I don't know about what, if any history, you have with your family and social services, nor what, if anything, they think your contribution with this may be.
So, I can't give an opinion here because I am going to be lacking a good many of the facts. However, what I would STRONGLY advise is that you make sure that both you and your daughter, especially her, are getting the support you both will need through all of this, even if social services were not involved. (Same goes for you getting the information and education you probably need per knowing how to be supportive of her yourself, especially at a time when you now have your own situation going on with this.) If you'd like for me to help connect you with local support services for both of you around this, please drop a line using the "contact us" link at the bottom of this page, and I will be happy to do that.
Editor & Founder, Scarleteen: Sex Ed for the Real World
Author, S.E.X.: The All-You-Need-to-Know Progressive Sexuality Guide to Get You Through High School and Col