Injury or Trauma: This is often the most common reason for spontaneous hand swelling, as it can result from a variety of minor to severe injuries, including sprains, strains, or fractures, which may not always be immediately apparent.

Infection (Cellulitis or Abscess): Bacterial infections can cause swelling, redness, and warmth in the affected area. Given the hand's exposure to potential pathogens, this is a plausible cause.
Allergic Reaction: An allergic reaction to an insect bite, sting, or contact with an allergen can lead to localized swelling.
Arthritis (Rheumatoid or Osteoarthritis): Certain types of arthritis can cause swelling in the hands, especially in the joints.

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 less common in the hands, DVT can occur and is a serious condition that requires prompt treatment to prevent pulmonary embolism.

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.
Complex Regional Pain Syndrome (CRPS): A condition characterized by persistent burning pain and swelling in an arm or leg after an injury or trauma.
Gout: Typically affects the feet but can cause swelling in the hands, especially in the joints, due to the accumulation of urate crystals.
Scleroderma: A group of diseases that result in the hardening and tightening of skin and connective tissues, which can lead to swelling in the early stages.