Loosie it has to do with the amount of power that the horse can easily generate from behind. That tail is WAY too low and croup too steep for any real engagement which sadly will be necessary to decrease the impact of his downhill build and upright pasterns.
Although your theory about posture vs structure does make sense, my horses both develop a rather similar stance when they are extremely uncomfortable. I have only witnessed it a couple of times but they DO do it. HOWEVER, this then leads us to infer that the horse must be uncomfortable or in pain, and therefore probably not sound. In the case of my horses, it was in the middle of nature's fury - and my filly's first storm. My gelding came from warmer climates and so the cold often brings this stance. It is more often pain causing such a 'tucked' appearance, like the horse has tipped his hips underneath him to stretch out (or relax) sore muscles. I would hazard a guess that this horse is either sore in the back or hindquarter, or tends towards a low-grade colic.
The unsoundness is really connecting dots considering when I (and other people who saw his video) evaluated his movement for the interested buyer, his back right leg did not look right at all. He had a really nice flat front movement but as he was loping and jogging he looked seriously uncomfortable. I'll see if I can post the video!
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