What would cause water to break as early as 31 weeks? is this usually causes by infection?

We don't know: We don't know why people have premature rupture of membranes. Sometimes infection can be the cause, but usually not. Infection can often occur after this happens, however, so this is considered a serious problem that usually requires a stay in the hospital.

Long list...: An infection is a common cause of premature rupture of membranes but it isn't the only cause. The list is pretty long and includes things like premature cervical dilation; an infection somewhere else in the body such as the kidney, appendix, lungs or teeth; recreational drug use; truama...etc. Fortunately we are in an era of very good NICU care. Best wishes!