Because she left several abusive, threatening voicemail messages on my teenage son's phone accusing him (wrongly) of bullying her son.
That was bad enough, but in between leaving these nasty voicemails, she was having a very normal, friendly conversation with me by text. Literally, she'd be texting me, then she'd call my son & abuse him. It was just a relief that his phone was off.
The two-facedness of it was unbelievable, and my son was very shaken by the messages. He let me & DH listen to them & we were shocked. We seriously considered reporting it to the police, and we did inform the school.
My son hadn't been bullying hers at all, but he had chosen to distance himself from the other boy because of the mother's constant interference & unpleasant, devious manipulative behaviour which he was finding difficult to deal with. I won't go into details but it was weird stuff, most of which we didn't know about until later.
I'm normally a very quiet, easy going person & I didn't do anything about it for a week or two but silently stewed. Also, my son didn't want me to do anything.
Then one day I bumped into her & she said "Oh, hello Drama, how are you?" as if nothing had ever happened. I'm sure she thought my son wouldn't have told me what had gone on. In that moment I saw red, confronted her on it and told her exactly what I thought of her.
I haven't spoken to her since.