General Grievous's cough is primarily caused by damage to his lungs sustained during a fight with Mace Windu. In the 2003 Clone Wars animated series, Mace Windu used the Force to crush Grievous's chest plate and internal organs, which led to his persistent, signature cough seen in "Revenge of the Sith." Additionally, George Lucas intended the cough to represent the imperfection of the cybernetic technology used to save Grievous's life after his near-fatal injuries. Grievous underwent extensive cybernetic enhancements that replaced many of his organic parts, including his lungs, which became irritated and malfunctioned due to the technology of that era. His cough highlights the mechanical vulnerabilities and incomplete nature of his cybernetic body, similar to how Darth Vader's breathing reflects his own cybernetic limitations. George Lucas even recorded the cough himself, using his own illness as the sound basis for the character's cough. So, the cough is a combination of injuries from combat and the flaws in the cybernetic technology that replaced his organic body parts, especially his lungs.
