Enamel hypoplasia occurs due to a disruption during the enamel matrix formation phase, which impairs the quantity and quality of enamel produced by ameloblasts. This disturbance can be caused by systemic factors such as illness, nutritional deficiencies, or trauma during tooth development. The defect manifests as pits, grooves, or thin enamel, reflecting incomplete mineralization or matrix formation. Since enamel is formed only during tooth development, any interruption during this formative stage results in permanent enamel defects. This contrasts with issues like fluorosis, which affect mineralization rather than matrix formation.

Reference: Oral Histology, Ten Cate, 8th Edition.