What could cause this strange double-image distortion?

Question

I pretty much ruined half my shots at this recent event and I'm not  sure how (I suspect operator error). The lens details are on the screenshots and the camera used was a  Nikon D4, center weighted metering, shutter priority. The shutter was in  rapid fire mode.  The first print screen shows most of the frame with  the main subject (the black dog & handler) having good focus and  clarity. 
The  problem is just outside the depth of field as seen in the 2nd capture.   In the below zoom it is easy to see the distortion I am talking about.   Notice that just outside the depth of field under the dog's belly is  what appears to be a "smear zone". Notice the double-blur of the string  and the blades of grass. A little further and the blur becomes  "normal".  This is really unsightly, and I have no idea what's causing  it:
It  looks like motion blur, but that cannot be since the dog is not blurred  and the shutter speed is high. This subject matter is fast moving and  hard to focus on but thats not the problem here.  I expect to see a gradual blurring away from the  focus plane, not this "zone" of double image.   This is one of my better  lenses so I cannot imagine it is the problem. I've shot this lens with a  lot of other subject matter and not seen this problem.  It has no VBR  so that cannot be malfunctioning. I note that it was very hot and humid that day in PA (heat  distortion?).  This is obviously baked into the photo (uncorrectable in  Lightroom) and all I can do is try to figure out what it is so I can  avoid it in the future. 
Any  speculation anyone can come up with would be very welcome, --not only  am I out of theories, I cannot duplicate this at will.  Attempts to  duplicate the problem in similar sunny conditions have failed.  My first  instinct is to call it diffraction distortion --but not at f/4  with that quality of lens!  And if that were the case it would be  repeatable.   If someone can at least give me some possible keywords, I  can do my own research.  Thank you in advance!

Answer 1

Thanks for the input. I was finally able to find some test conditions that duplicated the problem, and it is either that or some other out-of-spec diffraction distortion.  So I sent it off for repair.