One of my colleagues had this on his suit and wanted to post so as to offer some advice and/or garner suggestions as to how any tailors or seamstresses would fix it themselves if different from my view.
There are quite a few things that could be the cause in similar cases that you may have or have seen. When you see the parallel lines go from the collar on down over the shoulder blades of the back then most of the time it is probably too snug across the back.
This is not one of these cases. I believe that the cause is 2 things.
1. The colleague in question is rather muscular so the trapezius' and back muscles are well built and the shoulder can catch on these and create this bridge over a gorge effect.
2. The balance and/or length of the cloth at the back is off. Most people are pretty average (hence off the rack/peg clothing) but a good number of people are either more stooped or more straight postured than the coats will forgive.
When you have a jacket that has more fabric and is cut for a normal or stooped forward body and you are in fact more straight in your posture than you may get this. Most commonly affected are chaps under 5' 8" as they stand the straightest so as not to seem shorter than possible.
Take to your tailors and ask, if they are good, show photos if possible of the problem so you can be clear. They may not explain what they will do but they will probably either LOWER THE COLLAR 1/4" - 5/8" (max suggested) or SQUARE COLLAR. The alterations are different but hopefully will improve the look. In most very muscular chaps it would be rare to have a completely clean back here as they fabric is stretched over built up muscle which dip between the shoulder blades.