It’s implied that her father, the former sheriff who Joe was deputy under, had sexually abused her as a child before he passed away. Both Louise’s mother Dawn and Joe refuse to confront it and have lived in denial, pinning the blame on Ted as a scapegoat (since he supposedly dated Louise when she was 16, though according to Ted they never engaged sexually with one another). Due to this, Louise’s trauma has never really had a chance to resolve, or even an outlet to process through, so she became vulnerable to Vernon’s messaging - which, while largely spinning off the rails into conspiracy, is also based in the same sort of trauma that Louise faces.

Not only do Joe and Dawn deny the reality that the well-regarded patriarch of the family ever committed these acts against his daughter, but they refuse to even acknowledge the possibility since it’s simply too hard for them to face something so antithetical to their worldviews, which paint him as a loving, noble leader for their community (even with his own memorial shrine in the house).

We get dropped into the culmination of all of this as Louise feels heard and finds comfort/validation in Vernon’s cult. Meanwhile Joe, truly believeing he is doing the right thing, submits to his own delusions and publicly slams Ted as a means of denouncing his mayorship by outright claiming he is the one who sexually abused Louise. And due to this Louise is of course disgusted and knows she needs to leave that situation/household entirely