In Revenge of the Sith, the answer seemed to be the effects of Palpatine's Force lightning, which electrocuted him as he attempted to use it against Mace Windu. However, inconsistencies in Star ...

When Mace Windu ... to call Palpatine overconfident. "Your feeble skills are no match for the power of the dark side," Palpatine tells Luke as he unleashes a barrage of Force Lightning.