First Pregnancy loss at 18 weeks.

Got pregnant at 30. Everything went well with all ultrasound, fetal heart beat, and blood reports till 3rd month NT/NB scan. No issue detected. Suddenly by approaching my 18 week one evening I felt something bag-type coming out of my vagina while I went for toilet. I told my gynac. She called me to hospital and admitted me. She saw and told me the sac has come out. She asked me several times - you don't have any pain, I replied no. Put me to observation for next 2 days to check if the sac can go inside so that she caan put up a stitch but nothing happened. Insted the consecutive ultrasound saw that my water leaking and there is very less water for baby to survive which then turned out to inevitable termination of pregnancy. The doctor said - incompetance Cervix. I am still in mental trauma that this does not happen in first pregnancy normally or at least never heard from anyone. 1. Why my cervix got weaken? 2. Could there be "NO" option where doctor could have decected this before.