There is another aspect which probably isn't a big factor in modern ATX supplies but I will mention to be complete and that is where the 5V or 3.3V is underloaded and causes an issue. In older supplies there was often a primary regulation based on usually 5V loading. Low or no load on the 5V typically lowered what I will call the excitation of the power supply and basically it drops too low to provide enough energy to 12V circuits. Some of you may have noticed that we put a small loading on our PDB for 5V and 3.3V to avoid this issue.