Probably to teach her a lesson about having parties at home behind her parents back, worse having strangers over too.
One of my sisters had one of her daughters pull a stunt like this at the age of 17. Said niece wound up calling the police herself when a gigantic crowd showed up after the word spread very quickly that she was having a party while her parents were out of town.
This was about 25 years ago. With today’s cell phones and social media communication capabilities, I am sure such crowd problems are not too uncommon.