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Manager Guide: Lots of Unsubscriptions
Each list is set to remove member addresses from the list after they've been sent a number of "your membership is disabled" warning messages, according to the settings on http://lists.uua.org/mailman/admin/listname/bounce (see "Bounce action notification" for more about those settings).
In a nutshell, sometimes list traffic patterns and Mailman's system settings cause bad addresses to be discovered in bundles. The addresses that have been unsubscribed from your list have most likely been bad for quite some time, but the system only just "realized" it, and took action according to the list's settings.
Definitely alert web [at] uua [dot] org if you discover any of the email addresses are still good. Otherwise, assume that all is well!