Looking at that picture it looks like the problem is mostly due to the cut of the jacket. Unless the angle of the picture is throwing my perspective off, here is what I see:
1) The back of the jacket looks short. Almost a couple of inches. Again, it could be the photo. I have owned a few jackets where this is the case and that leads me to the next item;
2) The back panel not being as long as it should be means it weighs less. Combined with a light-weight leather like lamb or goat, the leather itself does not weigh enough to fall the way it should. I never have problems like this with a mid-weight cow, steerhide or horsehide.
3) Another factor may not be that the back is too short, but the balance of the jacket does not match your frame. When you stand up straight at your normal posture, the jacket should fall comfortably to the front and back. Does the jacket feel right on your shoulders? Or does it feel like it wants to slide backwards? Some jackets do this and when you pull them onto your shoulders so they feel right, the back will ride up like yours...
4) The jacket construction itself. A slippery slope, this subject.
a) The way that the components are constructed with reference you your physique. Not to say that anything is wrong with either. But they do not work as well as they could when combined. The jacket might look perfect on someone else with a slightly different physique.
b) The yoke panel seems low and that gives that infamous "weird bump" at the top of the action pleats. I never notice this on the shorter yoke jackets.
c) Jackets that measure the same circumference in the body, can look drastically different simply by moving the side seams and arm hole forward or backward. I have had jackets that looked tight across the chest, but the back panel has way too much material as KT stated. Of course, this works the other way as well. I have had jackets that feel like they are pulling across the back of my shoulders and are really tight, but the front of the jacket is roomy.
Moving the pattern of the armholes/sleeves forward an inch might have fixed this, but I am not a jacket manufacturer, so maybe I am way off base here. In your case, with your slim build, the lower back panel could probably be tapered a bit more to get rid of the extra material there...
d) The side strap is pulling (as you can see by the wrinkles). It looks as if the wrinkles start at the upper middle of the back and radiate downwards and out to each side strap (assumably they are the same on each side). You would think that the straps would pull horizontally and not at that angle.
That is based on this one photo though. You may find that as it breaks in and softens more, the jacket does not do that as much. How does the back look while zipped?
Do not listen to him kid......I once looked like you and was told the very exact same thing. Now I look like a donut in every which way!!!
