A patient's maxillary first molars have no antagonist (the upper arch only has teeth from 1st molar to 1st molar; the lower arch only from premolar/canine to premolar/canine), with cupping seen on all the upper and lower anterior teeth. What is the most likely cause of the cupping?

Cupping (cratered, scooped-out areas, often on incisal/occlusal surfaces) without a matching antagonist tooth to create wear-on-wear contact strongly suggests a chemical process — erosion — rather than mechanical attrition (which requires opposing tooth contact) or bruxism (also requires opposing contact/grinding). Abfraction specifically produces cervical wedge-shaped notches, not generalized cupping.