Jimmy instigated getting "fired". He was burned out and wanted to leave; however, he had years remaining on his contract and wanted to get paid for the entire contract.
If not for the contract issue, that scene where Jimmy, Jerry and Stephen were in crisis mode at Valley Ranch would never have happened. Jimmy would have just called in from Florida and told them he quit. He probably would have just told Jerry's secretary or Stephen because he was burned out on Jerry.
Jimmy's style caused him to burn out after being somewhere a few years. His style was also believed to only work with players for a few years before they also got burned out on Jimmy's intensity.