A critical challenge for operating fusion burning plasma in high-confinement mode lies in mitigating damage caused by edge-localized modes (ELMs). While impurity seeding has been experimentally validated as a reliable and effective ELM mitigation technique, its underlying physics remains insufficiently understood and requires further clarification. Through nonlinear magnetohydrodynamic simulations, this work reproduces key features of a natural ELM crash and reveals its trigger mechanism. Impurity seeding significantly affects nonlinear ELM dynamics by inducing local and global modifications to the pedestal pressure profile, driving high-n ballooning mode instabilities that govern an ELM crash. Two critical control parameters—impurity density level and poloidal seeding location—are systematically investigated, which play key roles in the timing of ELM crash onset and the resulting energy loss magnitude.