USAF schedules the lube, oil, filter changes for tractors at every 6 months or 400 hours. Annual preventive maintenance is 12 months or 1200 hours. So unless you're running your tractor with no lubricant in the rear end, racing at high speed down boulder strewn creek beds, or popping th clutch and yanking on heavy loads, there's absolutely no reason for it to have failed when it did.
Now the USAF rides tractors hard and puts them away wet. Usually they have 2 or 3 users a day in civil engineering squadrons doing normal tractor use in road and grounds. 6 to a dozen users per day if they are being used on the flightline towing equipment. So the life expectancy of their tractors is 15 years maximum if I remember from AF TO 36a-1-1301.