I don't know what happened in this case, but it's entirely possible that someone who wanted to bring them down organized some bot traffic to them. It happens f

I don't know what happened in this case, but it's entirely possible that someone who wanted to bring them down organized some bot traffic to them. It happens for much smaller mailing lists with far fewer enemies where attackers just sign up with trash email addresses in order to increase the chance of your mailing list to be flagged as a spammer account.