Trauma: This is often the most common cause of a hematoma, especially if there's a history of injury or accident. Even minor trauma can lead to hematoma formation, especially in individuals on anticoagulant therapy or with bleeding disorders.

Anticoagulant Therapy: Patients on anticoagulants (e.g., warfarin, heparin, direct oral anticoagulants) are at increased risk of developing hematomas due to their impaired ability to form blood clots.
Bleeding Disorders: Conditions like hemophilia A and B, von Willebrand disease, and platelet disorders can predispose individuals to hematoma formation due to defects in blood clotting.
Vascular Malformations: Arteriovenous malformations (AVMs) or other vascular anomalies can cause hematomas due to abnormal blood vessel structure and function.
Tumors: Both benign (e.g., hemangiomas) and malignant tumors can cause hematomas by eroding into blood vessels or disrupting normal tissue architecture.

Aneurysm or Arterial Rupture: Although less common, aneurysms or ruptures of major arteries can present as a hematoma and are medical emergencies requiring immediate intervention.
Infectious Processes: Abscesses or infected cysts can mimic hematomas and require prompt antibiotic treatment or drainage to prevent serious complications.
Hemorrhagic Transformation of Ischemic Stroke: In patients with a history of stroke, a hematoma could represent hemorrhagic transformation of an ischemic infarct, which significantly alters management and prognosis.

Vasculitis: Inflammatory diseases affecting blood vessels, such as giant cell arteritis or polyarteritis nodosa, can rarely cause hematomas.
Coagulopathy Due to Liver Disease: Severe liver dysfunction can lead to coagulopathy and an increased risk of hematoma formation.
Congenital Conditions: Certain rare congenital conditions, such as Ehlers-Danlos syndrome, can affect blood vessel integrity and increase the risk of hematoma.