I hate to admit why, but I know what probably happened. Sometimes, people use old-school "listserv" software for email distribution. That software can have an "interactive listserv" feature that does exactly what you describe: every reply is then distributed to everyone on the list. There is usually an address to unsubscribe to the list, but often people just hit "reply."
That's probably it. I've actually never had that happen to me on such a large scale before. It's sometimes happened when people use the bcc line for the email addresses, and someone replies to all.
After getting over my initial annoyance, it was kind of funny to see all the confused emails flying back and forth. I tried to email the ostensible sender directly, but that email bounced--mailbox full.
Commenting by HaloScan
