Why is my daughter's hair dreadlocking in the back?

i guess i'm a lil unclear as to what's happening with her hair.  your daughter's hair can't be dreadlocking over night...as that would be impossible.  if you're doing her hair in the morning by combing/finger-combing it, that would ultimately deter dreads.  the formation of dreads requires tangling and matting.  her hair can't matte if you're combing/styling it regularly.

with that said, what you should try doing is at night before she goes to bed, use a wide-tooth comb or your fingers to comb through her hair.  then instead allowing her to sleep on loose hair, create 4-6+ large sections of hair.  braid/plait (two-strand) each section.  what she'll end up with is 4-6+ large braids/plaits.  they aren't meant to look pretty or perfect.  it's just a means of securing her hair but there are added benefits.

if your daughter's hair is curly, then one way to discourage breakage and encourage growth is to choose styles, however temporary as these plaits/braids will be combed out in the morning, that will encourage the individual curl strands to work together.  when individual curl strands are going every which-a-way, you get tangles, knots and breakage. braiding/plaiting will encourage them to work together and make combing out her hair in the morning, easier.

plaiting/braiding her hair can help seal in moisture/oils.  her pillow and bedding can't soak up things that they're not exposed to.  so taking the time to plait/braid her hair at night protects it in the long run.

hope this helps