Trauma or Injury: This is often the most common reason for a swollen hand, resulting from accidents, sports injuries, or other forms of physical trauma. The swelling could be due to a fracture, sprain, or bruising.

Infection: Bacterial or fungal infections can cause swelling in the hand, especially if there's an open wound or cut that has become infected. Cellulitis, an infection of the skin and subcutaneous tissues, is a common example.
Arthritis: Conditions like osteoarthritis, rheumatoid arthritis, or psoriatic arthritis can lead to swelling in the hands due to inflammation of the joints.
Allergic Reaction: An allergic reaction to an insect bite, certain foods, or substances can cause swelling, redness, and itching in the hand.

Compartment Syndrome: A serious condition that occurs when pressure within the muscles builds to dangerous levels, cutting off blood flow and preventing nourishment and oxygen from reaching nerve and muscle cells. It requires immediate medical attention.
Necrotizing Fasciitis: A severe infection that destroys the tissue under the skin, including fat and the tissue covering the muscles (fascia). It's a medical emergency.
Deep Vein Thrombosis (DVT): Although more common in the legs, DVT can occur in the arms and hands, especially after trauma or prolonged immobilization. It poses a risk of pulmonary embolism if the clot breaks loose.

Sarcoidosis: A disease characterized by the growth of tiny collections of inflammatory cells (granulomas) in any part of the body — most commonly the lungs and lymph nodes — but can also affect the skin and joints, leading to swelling.
Gout: Typically affects the big toe but can also cause swelling in the hands, especially in advanced cases.
Complex Regional Pain Syndrome (CRPS): A condition characterized by persistent burning pain and swelling in one of the arms, legs, hands, or feet after an injury or trauma.