Working in the field of stress and strain in materials for 30 years, what first comes to mind it tension induced buckling. Where would that tension come from? Looking at the fine close stitching I would guess that the stitching is very tight and as the leather is softening the tension in the stitching is drawing the panels in. The upper chest symmetric creases look to be a result of both the shoulder stitching and the arm stitching drawing in the softening leather. The back panels also seem to be affected by this same tight stitching overcoming the leather. The arms might be freed from this phenomenon due to the amount of flexing they receive from normal wear possibly loosening the stitching and reaching an equilibrium with the leather.
Thanks for posting this, that is very interesting and I think you could very well be right. It's definitely the best explanation I've heard so far. When I got it 'triple stitched' didn't really mean anything to me, but what you've described fits with how it feels on my back. I put it on this morning, over just a shirt, and seems it's a struggle to get the jacket sleeves to sit comfortably over a shirt's, when that was never the case to begin with. So perhaps the sleeves have suffered the same fate.
Hard for me to draw any reliable conclusions at this stage, as I'm sure you appreciate.
But I wonder, do you think this type of stitching could be responsible for the fantastic grain that's popped out all over the jacket? The grain, creases and wrinkling look better than I've ever seen and if this is related to the stitching then that would be one effect that is desirable.