Users being 'automatically' added to the deny list - has this happened to anyone else?

Within the past week, users have been added to the deny list seemingly automatically. I am the only admin that physically adds users to the deny list. And I know I have not added these users -- three in all so far. Before you dis this as administrator error, read on...

The last major event performed before this began happening was that I created a sandbox of an application. 

The next day, some users were added to the deny list the very morning of their last access. They are not users of the sandbox app. 

I have had one regular user get added to the deny list this morning whose last access was yesterday. 

QB Support has asked me to submit this to the forum. Has this happened to anyone else? If so, have you discovered the cause?

Thanks,
Chris