Tenderness on percussion of a tooth before root canal therapy is most commonly due to inflammation of the periodontal ligament (PDL). This inflammation occurs as a response to irritants from the infected pulp or periapical tissues, leading to increased pressure and sensitivity in the PDL space. Percussion tests specifically assess the status of the PDL, and tenderness indicates that the ligament is inflamed rather than the pulp being necrotic. This finding helps differentiate symptomatic apical periodontitis from other conditions like pulpal necrosis or cyst formation, guiding appropriate treatment planning.

Reference: Pathways of the Pulp, 12th Edition.