Trauma or Injury: This is often the most common reason for swelling in one hand, as it can result from a variety of injuries such as a fall, a blow to the hand, or overuse. The swelling could be due to bruising, fracture, or sprain.

Infection: Bacterial or fungal infections can cause swelling, redness, and warmth in the hand. This could be due to a cut or puncture wound that became infected.
Inflammatory Conditions: Conditions like rheumatoid arthritis or psoriatic arthritis can cause swelling in the hands, although they typically affect both hands.
Venous or Lymphatic Obstruction: A blockage in the veins or lymphatic vessels can lead to swelling, although this is more common in the legs, it can occur in the arms.

Deep Vein Thrombosis (DVT): Although less common in the arms than in the legs, DVT can cause swelling and requires immediate medical attention to prevent pulmonary embolism.
Compartment Syndrome: A serious condition resulting from increased pressure within a muscle compartment, which can lead to nerve and muscle damage if not promptly treated.
Necrotizing Fasciitis: A severe infection that can cause swelling, pain, and redness, and can rapidly progress to a life-threatening condition.

Sarcoidosis: A condition that can cause swelling in the hands due to granulomatous inflammation, although it is rare and typically involves other parts of the body.
Amyloidosis: A group of diseases that can cause swelling due to the deposition of abnormal proteins in tissues, including the hands, but this is exceedingly rare.
Klippel-Trenaunay Syndrome: A rare congenital condition characterized by abnormal development of blood vessels, which can lead to swelling and other symptoms in the affected limb.