The problem is password reminders are sent from the site list. This is the case because the reminder is for all lists in the domain, not for a single list.
In the past, the bounce would return to the site list -bounces address where it would be ignored because the bouncing address was not a member of the site list.
There were also other issues relating to bounces of list owner notifications sent from the site list.
Thus, handling of site list bounces changed in 2.1.10 and again in 2.1.11 and now all site list bounces are just sent to the site list owner.
Regarding password reminders, assuming bounce processing is set appropriately for the regular lists, what happens is a list member sets delivery off and ultimately forgets about the list. Then the member address dies and the monthly password reminder bounces. In the past, this bounce was effectively ignored, and the reminders continued to be sent and bounce forever.
Now, at least you see the bounces and the appropriate thing to do is manually remove them from the list(s) or turn on their list delivery so the list's bounce processing will remove them. Once you do this for the initial batch you see after upgrading to 2.1.11+, you will see very few going forward.
For more information, read the blocks of comments in the definitions of the __init__() and _dispose() methods in Mailman/Queue/BounceRunner.py.