Polyethylene (PE) is widely used as a liner material in high-pressure composite hydrogen storage tanks. During routine operation or maintenance, gas decompression occurs, resulting in pressure differentials within the polymer that lead to degradation or even failure if the rate of decompression is too high. Therefore, a comprehensive, fundamental understanding of the failure mechanisms of PE during rapid decompression is important to guide the development of safe operating procedures. In this study, atomistic simulations are employed to investigate the microstructural changes in amorphous PE during rapid decompression, under the influence of pressure, temperature, and stress. It is shown that the solubility of hydrogen in amorphous PE increases with both temperature and pressure, as observed in experimental work in the prior literature, exhibiting dual-mode sorption behaviour above the glass transition temperature. As decompression starts, the fractional free volume (FFV) within PE is observed to increase, with a corresponding increase in the number and size of free volume pores; it is hypothesised that this signifies the onset of cavitation damage. An increase in hydrogen content and initial pressure significantly increases free volume generation druing decompression, thereby increasing the risk of cavitation. Higher temperatures and tensile stresses also contribute to an increase in free volume generation. On a molecular level, the mechanism of rapid decompression involves volumetric expansion, leading to increased internal free volume, with hydrogen molecules further facilitating free volume generation. These molecular insights contribute to a deeper understanding of the failure mechanisms of PE, helping predict material degradation and failure during high-pressure hydrogen decompression.