Nothing I do seems to get it to do that, and it would be nice to have the achievement.

You give him back an item he gave you. Pisses him off.

it seems he's not a fan of items w/ the word wreath in it.

and , this gets ME burned, but does not result in the Red Rreign harming my pets. Is this a random thing, or am I glitched?

It's a random thing, unfortunately. He also goes after items in your inventory, so don't keep anything expensive in there.

Yes, this is the answer, thank you! I would have one window open to my shop which has all my extra "wreathed items" and then I removed them one at a time to be burned in a second window.

He hates Wreathed and Angel Rreing plushies or beanbags :)