As you can see in the picture, your arm has outer veins and deeper veins. A set of outer veins run along the thumb side of your arm, and another set runs along the pinkie side. Eventually, they unite to form one main outer vein.
Another set of deep veins drain blood from the deeper areas of your forearm and go up towards the armpit. The deeper veins join together to form one large vein that drains all the blood from from arm into your chest. Just before going inside the chest, this large vein also gets blood from the main outer vein.
Blood clots in the arm mostly happen at the main vein draining blood into the chest. They may also happen inside the chest after draining blood from the whole arm. The main vein has to cross a few structures before going inside the chest. This is a vulnerable spot for the vein. If the path is too narrow, blood flow may be restricted. This vein might be pinched by bones or squeezed by muscles.
Some people are born with an extra pair of ribs. This extra pair grows out of the neck spine and narrows the already tight space. People with this extra pair of neck ribs are at a higher risk of getting a blood clot in the arm.
Some people are born with extra muscles in the narrow area that may compress the vein.
In other people, the narrowing of the area happens after birth. People who have had fractures or surgeries of the collar bone or first rib may have abnormal growth spurs of bones when healing. People who perform repeated and strenuous tasks while holding their arms above their heads may have an overgrowth of muscles, as shown in the picture. The overgrown muscle may press on the vein and restrict blood flow. Repetitive and persistent overhead arm movement may impede the flow of blood even without any permanent changes, helping form a clot in the vein that drains blood from the arm to the chest.
Athletes whose jobs involve strenuous and repetitive overhead arm movement, such as baseball pitching, frequently have blood clots in their arms.