Increased mobility of a tooth is primarily due to an increased width of the periodontal ligament (PDL). The PDL acts as a shock absorber between the tooth and alveolar bone, and when its width increases, it indicates a loss of attachment or trauma, allowing greater tooth movement. This expansion often occurs in response to inflammation, trauma, or occlusal overload, leading to decreased stability. Therefore, an expanded PDL space correlates directly with increased tooth mobility. Other factors like cementum changes or no change in PDL width do not significantly influence mobility to the same extent.

Reference: Orban's Oral Histology, Ash & Ash, 14th Edition.