People do all sorts of things for amazingly petty reasons. Often they react out of proportion to the situation.
But in this case, I know there's more to the story than just "your mom made you move with her new husband". First, there's adolescence, that time in your life when you're trying to figure out how to be an adult but still something of a child. A child-parent relationship can be strained enough at times, but because there is a history, a bond, between them, most of the time these strains can be endured. However when one parent is replaced with an adult, a step-parent, that history or bond is gone; that new adult doesn't have the patience or interest in the child that allows them to know when to hold fast on the rules, when to punish, & when to try to make amends. My step-mother made many of those mistakes, & she still doesn't realize that she screwed up, let alone ponder how things could have gone better.
Further, adolescence is the time of one's life when everything takes on Great Importance. We are swept up in causes, everything is black & white, all the while plagued with private doubts that we are not smart enough or strong enough or cool enough to live up to our own expectations. When Kimberly/Lori cut the bonds between her & her family, it was Final & she could never go back. (Well, she could if she wanted to admit she was wrong, or even worse, give up, but that would be worse than handling the pain that act doubtlessly caused. And I bet she weighed the consequences of contacting her family many times afterwards.)
Also, one issue in the relationship between the child & a step parent is the remaining parent: they are caught between supporting their spouse & defending the child. Some just avoid the problem entirely. Some try to balance the two & fail. There is a reference to "new rules, a new school, and at some point, it became too much for Kimberly" -- which undoubtedly masks a lot of conflict. (I notice there is no mention of the step parent still in Kimberly's mother's life. Maybe it is significant -- or not. I know that, despite the failures of my stepmother towards me & my sister, my Dad is still married to her after over 40 years.) So even if Kimberly's mother knows the reason she cut off all contact, we'll never know it. There are some things people don't want to share with a world of strangers.
Which is why I'll stop with my self-indulgent rant here.
