Simple issue, you’re in your inventory and queuing for a dungeon or arena. You’re done with your inventory/settings menu/some other menu and you click ESC to get out of it. The queue pops and your ESC click just declined it and you got penalized. Now you can’t queue for the next 15 minutes.
One simple work around is to not use ESC when queuing for something but implementing a setting so that ESC is disabled for matchmaking popups would help dramatically because its hard to change habits.