Despite all the possible ways your arm vein can get compressed while going from arm to chest, the number of people who actually get blood clots in the arm is very low. That is because there is one important factor that decides whether or not you get blood clots: the coagulability of your blood. The coagulability of the blood measures how easily your blood forms blood clots.
Making blood clots are one of the defensive mechanisms of our body. People get cuts and scratches all the time. Our body stops the bleeding by making clots. Without the ability to make blood clots, we could bleed to death from a minor cut. Some people have an overactive blood clotting tendency. Those people form clots very easily. An area of turbulent or decreased blood flow may trigger the clotting mechanism, and blood clots may form inside blood vessels, causing a blockage.
People who get blood clots in the arm usually have a greater tendency to form blood clots. If you happen to be a baseball pitcher with a higher tendency to make blood clots, you may get blood clots in your arms. Not all baseball pitchers have clotting tendencies.