1. It's not a listserv. It's a mailing list. ListServ is obsolete, expensive, closed-source garbage software used exclusively by people who don't know any better and like to waste their money.
2. This problem was possibly, but not certainly, caused by a misfiring cron job. (I've seen it before.) The list-owner(s) were probably made aware of it almost immediately, because on a mailing list of this size, there are always SMTP rejects of some small fraction of the monthly reminders -- and this time, they probably arrived in batches of 16. ---rsk