As far as I'm aware, it can be caused by various things, some of which are inter-related:
- Jacket being too tight across the back of the shoulders;
- Too tight across the top of the shoulders;
- An erect stature;
- Square shoulders (similar to erect stature); or
- A short back balance.
Generally, tailors will try to rectify the horizontal wrinkle below the collar by taking up the excess cloth in the wrinkle and putting it under the collar.
Thank you. I guess I need to layoff some more pounds. It cannot be from doing shoulder presses for just 1 month or so.