My condolences for your loss. A midtrimester loss due to early rupture of membranes is a very difficult thing. Sometimes we don't always learn the underlying cause, but very often it is due to an infection. Cervical incompetence (or insufficiency) is another possible cause, however, you do not fit the profile of that type of patient given your 3 prior term deliveries (unless you have had a procedure to the cervix in the interim).
Review with your physician your complete medical history and discuss the pathology and any other findings. Unfortunately, a clear cause may not be determined.
oh my I am so sad for your loss.