Hematomas can occur after surgery if blood vessels at the site open as a result of heavy pressure or physical contact, according to BreastCancer.org. However, the most common cause of a hematoma is trauma, and sources can range from severe car accidents to the sudden twisting of limbs, notes MedicineNet.
The controlling factor in the formation of a hematoma is damage to a blood vessel. Blood leaks from the vessel into the nearby tissue, and over time it starts clotting. The more bleeding that occurs, the larger the clot that develops. People taking blood thinner medications have increased risk of hematomas, because the body lacks the ability to fix blood vessels efficiently, causing ongoing leaks of blood and subsequent hematoma formation, according to MedicineNet.