Cellulitis: This is the most common cause of periorbital soft tissue swelling, often resulting from bacterial infection, typically Staphylococcus aureus or Streptococcus pyogenes. The infection can spread from the skin, sinuses, or from an insect bite.

Allergic Reaction: Allergic reactions, including angioedema, can cause periorbital swelling. This could be due to insect bites, certain foods, or medications.
Trauma: Blunt trauma to the face can cause periorbital swelling due to hematoma or edema.
Blepharitis: Inflammation of the eyelids can cause swelling, redness, and irritation, often associated with meibomian gland dysfunction or seborrheic dermatitis.
Conjunctivitis: Inflammation or infection of the conjunctiva can cause redness, itching, and swelling of the periorbital area.

Orbital Cellulitis: A serious infection of the orbital tissues posterior to the orbital septum, which can lead to severe complications, including vision loss, if not promptly treated.
Cavernous Sinus Thrombosis: A rare but potentially life-threatening condition that can occur as a complication of facial or sinus infections, characterized by thrombosis of the cavernous sinus.
Herpes Zoster Ophthalmicus: Reactivation of varicella-zoster virus in the trigeminal nerve ganglion, which can cause severe pain, swelling, and potentially lead to complications like vision loss if not treated.

Wegener's Granulomatosis (Granulomatosis with Polyangiitis): A rare autoimmune disorder that can cause inflammation of the blood vessels, including those in the periorbital area, leading to swelling and other symptoms.
Sarcoidosis: A condition characterized by the growth of granulomas (inflammatory cells) in various parts of the body, including the eyes, which can cause periorbital swelling among other symptoms.
Thyroid Ophthalmopathy: A condition associated with Graves' disease, where swelling of the tissues around the eye, including the eyelids, can occur due to autoimmune inflammation.