Note that he graduated the Naval Academy (and was commissioned) in the first half of 1989... and left the USMC in the first half of 1995.
That's 6 years on active duty, so the only issue would be his flying status.
As I have known a number of aircrew whose medical clearance was pulled through no fault of his own, I find it highly insulting to the Corps that the first thing you Brits seem capable of considering is bad conduct or some nefarious side-deal... with "down-checking yourself" as the only other option mentioned.
I spent 6+ years fixing the things, and most of the pilots who left early were due to medical issues that developed after they were with the squadron... from one who developed "bends" at altitude to the one who suffered a detached retina during high-G maneuvering to recurrent virtigo and other unexpected problems.