I am not sure this will help but it does sound a lot like what is being described.
A few weeks ago when I first turn back on an old S5, it got an IP address from my router that was assigned as the DMZ IP address. I did not realize this happened until sometime later.
Basically that meant the S5 was exposed to the open internet. When I went to check the miner and pools I saw a entry on there for viabtc that I did not add!
Rebooting and removing the pool would only stick for a little while before it was added again.
To get it fixed I had to completely reset the device, take it off the DMZ (once I realized it was there) and reenter all the pools I wanted.
It has been fine since then.
My theory is that someone is running a bot that connects to open Antminers using SSH and the "Antbleed" API and setting their pools whenever they find a vulnerable miner (like mine was).
My lesson is never expose you miner to the open Internet!