Why would someone go and tattle on my mom to the Roman Catholic Church?

Mom was raised Catholic, and grew up in the Pacific Northwest. Huge family.
She met and married my dad, who was from Texas. If he had to claim any religion it would be Casually Methodist.
One thing led to another, and ultimately we all wound up in Kansas, far away from her Catholic family in the Pacific Northwest.

After they’d been married 10 years, and had 3 kids, Mom received a letter from The Church stating that they did not recognize her marriage because Dad had been married for about 6 months, right after high school. They excommunicated her.

For the church to find all this out, someone, a family member up north, had to have tattled.

Why the hell would anyone do that?

BTW, she told the church to go to hell, and walked away from that religion. It really shook her up, though. I think it made her question everything, but she never shared it with us.