Quantum mechanics predicts measurement outcomes with remarkable accuracy, yet it does not identify the physical mechanism that produces them. Standard formalisms treat collapse as an external postulate or an informational update, leaving the origin of measurement undefined. This paper develops a substrate-based account in which measurement arises from the structural properties of mass rather than from observation, information, or consciousness. Within the Quantum Substrate Dynamics (QSD) framework, all stable matter possesses a finite coherence envelope and participates in discrete Causality Intervals (CIs) that bound how the substrate can reconfigure. Massless excitations such as photons do not possess coherence envelopes and therefore cannot induce or modify collapse; they only traverse the geometric constraints imposed by nearby mass-phase structures.From this asymmetry, a coherent picture of measurement emerges. Collapse occurs when the envelope of a mass-phase structure overlaps a propagating excitation and enforces local CI pacing and curvature--compliance limits, defining which outcomes are structurally supportable. This mechanism reproduces the behavior of detectors, the material dependence of diffraction, the emergence of interference patterns only at mass-phase boundaries, and the well-established fact that light beams in free space do not interact. Classical paradoxes: Wigner's friend, Schrodinger's cat, contextuality, and delayed-choice interference are resolved without invoking global wavefunctions, observer-dependent realities, or branching worlds.The result is a fully physical, mass-induced account of quantum measurement. Collapse becomes a structural re-lock within the substrate rather than an epistemic update or interpretational supplement. This framework preserves the successful predictions of quantum theory while supplying the missing mechanism: observation is not a special act, but the structural consequence of mass interacting with serialized, massless propagations under finite substrate throughput.