Since they suspended his supervisor as well over this I think it has to do with him retiring in 2 years more then the insubordination and unauthorized access.

John Chevilott found a gun, secured it, brought it back to the road yard in Westland, which is Wayne County property, before he turned it into police. That seems to be where the problem lies because right now in Wayne County there is not a policy describing what to do if an employee finds a gun on the job.
However, Chevilott's superiors at the Department of Public Services had a much different opinion. His foreman, who had knowledge of the situation, was suspended for 30 days, and after 23 years on the job, Chevilott was fired for violating department policies.

According to a Wayne County spokeswoman and the rules, employees aren't allowed to possess a weapon on work property.
Chevilott also was let go for insubordination and unauthorized access to the road yard.