In classical caesarean section more chances of rupture of uterus is in:

Upper uterine segment is at more risk of rupture in case of a previous classical cesarean section. Here incision is made on the anterior uterine wall in the upper segment above the reflection of uterovesical fold of peritoneum. It is because such a scar is difficult to appose and healing is not good because of frequent contractions and retraction during puerperium. (Sheila TB of obstetrics 2nd edtn pg 338)