I did some more reading after I posted and you are right... the blurriness of backgrounds is the human brain's inability to adapt to the projected 3D.
That's not the effect I was mentioning. Pause it. Look at the foreground. It's blurry because it's deliberately out of focus. Look at the people, 100% in focus. Look at the background. Slightly out of focus. This is how they build movies in 2D to train your eye to look at the object of focus. But in 3D, the blurry object is in front. Your focus is drawn to the object in front. But it's blurry. Secondarily is then the effect you mention where then the object of focus is out o focus for the viewer.
But the physiology of it is that the rapid movement of focus by the viewer to try to follow the movie is tiresome to the eyes, and can lead to fatigue, headaches, and possibly eye damage in under-13s.
The directors of 3D movies are sill directing to have the movie shown in 2D (as more 3D movies are seen in 2D than 3D), so they use 2D visual effects that are incompatible with 3D. This negative feedback loop makes 3D viewing worse, driving more people to watch 3D movies in 2D, so more 2D visualization is required.
Also when I can't bring a background image into focus it breaks my suspension of disbelief in a movie.
That effect has always been there in 2D, where the front and back aren't both in focus. But in 2D, you recognize it as part of the limitations of the media, and, so long as the object of focus takes your attention, you don't notice or care. But 3D uses a different mechanism for objects of focus, and should use different tricks. But they don't, because the same final product is shown 2D and 3D.