Sophia Hess's trigger event came from being in her stepfather Steven's car when he was killed in a gang hit.

We know that Breaker triggers generally come from situational threats, and being in a wreck of car would certainly qualify. A gang hit seems likely to have sparked Sophia's "predator versus prey" mindset, if she felt that Steven only died because he was "weak" (people certainly come up with worse justifications for random deaths after all). It could be that because of her trigger, Sophia never moved on from the Anger stage of mourning. It would make sense that she hates Taylor so much (and considers her "weak"), because Taylor went through the same emotional trauma of losing a parent figure and didn't trigger.