Something weird happened when the list host changed. For some reason I had stopped getting mail from the list for quite some time. Then boom, the messages started coming again. (I sub to too many lists, so I sort and often ignore). I suspect something happened that 'dormant' subscribers got reactivated. They were surprised. Many unsubbed. So I think it may not just be a case of clueless users, but something triggered in the system to send mail to folks who probably did not think they were on the list.
if an account bounces a lot of mail (say for an exceeded quota) over a period of time, Mailman will set that individual to 'nomail' rather than continuing to make delivery attempts. most likely a number of people were "reactivated" by the list move.