Because BPD.

Possibly they don't want to be alone at times, so they reach out to anybody they think is willing to spend the night, even you.
Another possibility is the classic push/pull behavior. In that case they may really want nothing more to do with you, but keep testing you to see if you still want them.
Ah, yes... .the good ol' push/pull. I am extremely LC with my ex now and she has started that with me. It's emotionally confusing (and trying). I have been strong so far. Even though I desperately want to be with her again (both emotionally and physically speaking), I know that I can't. Still, that doesn't make it any easier.
It has been awhile since her and I had the final talk. I've slowly been detaching more and more and going more LC with each passing day. She still has moments where she asks me why I'm distancing or says something about how "the good days aren't lost forever" and so on. It's almost like she doesnt accept that the r/s is over, even though I know she's out doing things she 'shouldn't be' if we were in a true r/s.
Good ol' BPD in action.