Comment 1 for bug 930462

The members were almost certainly unsubscribed by Mailman's bounce processing. Your hosting provider can review Mailman's 'bounce' log to verify this and can examine the logs of the outgoing MTA to possibly find out more.

You can review your list's bounce processing settings for appropriateness and ensure that all the owner notifications are turned on so that you will receive notification when a member's delivery is disabled by bounce and before the member is removed.

Without additional information to indicate that this is actually a bug and not a result of normal bounce processing, I can only assume that this is not a bug.