What would drive a mother, hell any parent to bully their son or daughter relentlessly?

Before you answer this read the description..

There is a documentary on NETFLIX called "unknown number: the high school catfish" where this young girl and young guy that were getting relentlessly bullied by unknown numbers.

TLDR it turns out the young girls mother was the one bullying both her and her boyfriend.

My question is why in the fuck would any parent, never mind gender, do that to their kids and their kids significant other?
What would drive a mother, hell any parent to bully their son or daughter relentlessly?
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