The same thing has been happening to me. I have complained repeatedly to Yahoo about it. It seems that whenever an email from a group to a person bounces (they claim it has to have been happening for several days, but I don't believe them), they unsubscribe the person from *all* their Yahoogroups.
This policy is unreasonable and should be changed. If a person is still receiving some of their Yahoogroup emails (something they should be able to check without too much difficulty), Yahoo should assume that messages are being inadvertently rejected as spam due to filters or what-not. In my case, I look through my "junk" email folder carefully before discarding anything. Occasionally I have found a Yahoogroup message got filtered, and I mark it as "Not Junk" and don't bounce it back as I do with the rest of the spam. While it's possible I've missed an occasional message, I think this is a much rarer occurrence than the frequency with which Yahoo unsubscribes me from my groups. The only other answer I can think of is that my Internet Service Provider, Earthlink, is filtering some of the emails as spam before they reach me. But Michael doesn't use Earthlink, and if Earthlink is indeed doing this, then it is affecting *lots* of other people because Earthlink is I think something like the fourth-largest ISP, and Yahoo should act accordingly and stop inconveniencing individuals.
Love & Liberty,
<<< starchild >>>
P.S. to Yahoo - Please note that I have had to manually delete the advertising spam that you have started inserting as sidebars to each message posted on Yahoogroups. These sidebars have screwed up the formatting of my emails (I use Mail for Mac 10.4), and it is a real pain. Please stop using them! Stuff like this is reducing the value of Yahoogroups to those of us who use them.