It really depends on the thrower but with most it is actually related to footwork, weight shift and body positions that keep your shoulder "up".
If the shoulder plane is pointing up, the disc will go that direction. A lot of this can be solved with some simple changes. First is make sure your body weight is moving with the disc at all times. In most cases the shoulder pops up when the weight stops on the back foot and the arm accelerates without body weight. You need to work on driving the weight through with the disc to keep the shoulder plane level or slight down. If your shoulders point up or rotate too fast and your feet are flat - the disc will come out early, late, up, and every other bad way based on how far off you are. If you see the disc coming out flat and to the right, you are on the right track, you just need to slow down your shoulders.
Bad habits will be formed if you try to counter act the shoulder up by throwing your arm in a downward direction and/or forcing it over/down to make it turn flatter. Usually leads to injury.
Here is a simple drill I use to help people:
Stand flat footed, knees locked and in a basic throwing positions. Try to rock your weight forward while flat footed. You will notice that you have to kind of "raise up" over the center of your body in order to move forward. This is what causes the shoulder to pop up and the disc to go SKY HIGH.
Now get back in the same throwing position, but bend your knees and get on your toes as best as possible. Try to **** forward with the disc. You will see it is much easier to maintain a flat shoulder plane and rotate the hips forward/open. These simple things are the issues with 99% of the people I teach. The arm stuff can be figured out later. Get the lower body in check first. The reachback and arm angles are easier to fix than this.