Diocletian and Maximilian Abdicated at the Same Time

Both of the caesars needed to become augusti at the same time. The trick was getting maximian to go along with the plan in order to insure that the balance of power remained intact as authority was handed from one generation to the next. So it is entirely possible that the illness was just a coincidental stop on the road to cementing a precedent of 20 year long reigns for all future emperors.