What could cause frequent sinus infections?

Multiple things: Frequent colds (uri), nasal allergies, hay fever, smoking, nasal polyps, septal deviation, turbinate enlargement, and foreign material in the nose (common in kids) can lead to sinus infections.

Allergies or nasal: Recurring sinus infections can be caused by frequent exposure to others who have respiratory infections(ie, teachers, students) ;or having underlying allergies; have some "anatomic" issues with your nasal and sinus passages; or rarely a "minor" immune problem. See an allergist for a careful evaluation. There are many good treatment options.

Limit exposures: Several things contribute to chronic sinus problems. Most issues are not truly infections, but usually more congestion. Avoid smoking and smoke exposure, explore allergies, consider nasal saline irrigation, and avoiding dairy products for 2 weeks (most adults have some version of milk allergy).Sometimes, recurrent issues can be from lacking some immunologic factors(iga, etc) may help identify.