It's happening because your game clients are changing resolution in such a way that the game wants to force a reload of the UI. This isn't something that is new with patch 7.0, but I guess Blizzard decided to expand upon it—I imagine there's something new in the UI which requires this.

If you want to avoid it, then I assume you could always just set your game clients to be the same resolution, or, if they're not already, use resolutions with the same aspect ratio.