There are a few different things that can cause final drive failure. One of the most common causes is simply wear and tear. Over time, the gears in the final drive will start to wear down, causing them to slip and eventually fail. Another common cause of final drive failure is contamination. If dirt or other debris gets into the gearbox, it can cause the gears to grind and eventually fail. Finally, if the bearings in the final drive are not properly lubricated, they can seize up and cause the drive to fail.