How does chorioamnionitis cause preterm birth? isnt the fetus protected by the amniotic sac and the mucus plug so that bacteria wont make its way up to the uterus and the membranes?

By definition a chorioamnionitis occurs when bacteria have bypassed the mucus plug, invaded the amniotic sac and triggered infection/inflammation. That inflammation triggers expulsion of the fetal/amniotic sac contents. Premature rupture of the fetal membranes is the presumptive trigger. Rupture past 18 hours has a high rate of chorioamnionitis, progressing by the hour.