Rapid decompression failure is a significant challenge for polymer liners in high-pressure hydrogen storage systems, driven by cavitation-induced damage. Understanding the mechanisms of cavitation initiation is critical for improving material performance and ensuring operational safety. This study investigates the behaviour of amorphous polyethylene (PE) under rapid decompression using molecular dynamics (MD) simulations. Amorphous PE models with preloaded hydrogen concentrations are constructed based on solubility data from Grand Canonical Monte Carlo (GCMC) simulations. These models are subjected to decompression from 700 atm to 1 atm at 300 K, and the evolution of free volume is analysed to identify conditions that promote cavitation. The results show a substantial increase in fractional free volume (FFV) during decompression, with an increase in pore size, and a rise in the number of cavities. New free volume regions are observed to form, attributed to the redistribution and motion of polymer chains, indicating that cavitation is not solely dependent on pre-existing voids. The findings provide atomistic insights into the mechanisms of free volume generation and cavitation initiation. This study contributes to the design of safer and more durable hydrogen storage systems by addressing critical gaps in the understanding of polymer liner behaviour under extreme conditions.