My waters broke at just over 38 weeks, I had a 9lb baby. The midwives said that it was probably due to the pressure of my constant sneezing, coughing and nose-blowing (I had the flu) that caused the pressure, and then the final nose-blow (when my waters broke) just tipped it. My DD wasn't engaged when my waters broke, either. Also, I wasn't dilated at all.

I think it's just one of those things that can't really be helped, but once it happens its best to move around to help get the labour going so you don't end up needing the gel or other induction methods.