I've seen it many times over the years , but I think it's just pent up energy that was to be used in order to alleviate internal 'meltdown'. They do it when they are inspired to.. I've seen it happen with a blowing leaf in the middle of the day... or a simple lure toss, or any other motivator. The funniest ones for me were when we raised Ricky , a gos in 2006. He would quietly be sitting on the perch or floor, and Oreo the kitten he was being raised with would walk into the room.. One second he would be standing there, and the next second they would be on the floor wrestling WWF style no holds barred. Oreo would have him in a head locked kicking away while Ricky would have both feet footing the crap out of the kitten... along with many a bite and wing whipping. If you didn't know any better it would appear as if it was all out war , but they were just playing... After a bit of that, one or the other would either run off or collapse on the floor. The other sensing victory, would lay down as if spent and fall asleep.
BTW the picture of the kitten under the perch was at around 90 days , fully a month aftyer hard penning..... Ricky got killed by a RT on a long flight on a pheasant kill a month or so later.
"you believe you understand what I said, do realize what you heard is not what I meant"
Barry