What causes any type of cancer? i know it's cells dividing uncontrollably but why does that happen?

Mutations: A clone of cells has acquired, through natural selection, several genetic mutations each giving an unfair growth advantage over the neighbors. When the genome itself has become destabilized, more mutations accumulate rapidly & the cells recovery the primitive ability to invade and spread. The best way to whether a "pop" cancer-cause claim could be valid is if the exposure is a powerful mutagen.