Things that came to mind Mark as I watched your video, were, mites infesting the actual straw (not chook mites) but which do crawl on chooks so the chooks want to clear it out; or they are trying to find coolth in this extra hot season & are misguidedly scratching in their boxes rather than digging in the ground; or trying to turn the thick barley/oaten straw into finer stuff to make a better nest with.
I had one of my chooks ripping up the paper I had in there. They did it several times, while I was away and my daughter in law had to sort it out. I came home and gave it a good telling off and it stopped Maybe it just missed me.
I think chooks just like to tick you off sometimes