you are burning belts in reverse because you are loading the engine up against the reverse limiter. you hear it tagging the soft rev limiter, means the engine is cutting out and dying. every time your engine dies, about 2 times a second, the clutch releases the belt. the clutch is torque sensitive, as you apply torque, the clutch closes and grabs the belt. when you're hitting that rev limiter (around 4-5mph in reverse or 3500rpm) then the engine is constantly dying, over and over and over, and the clutch is releaseing the belt, and when the engine fires back up the clutch grabs the belt again over and over and over. the clutch is opening and closing as fast as you are hitting that rev limiter 2 times a second, this is making the clutch slap the belt over and over and over and will not apply a full constant grab to the belt so it makes the clutch slip the belt very easily.
you gotta remember to hold the reverse over-ride switch when backing up in the mud. i slipped a belt earlier today on a mverick backing it up a small hill on my own place with the owner riding shotgun. i tell folks to do it all the time, dont back up under loads without hitting the reverse over-ride button or it will slip a belt. and i did it myself today backing up turning around on flat ground but had the back end kinda drive up over a little hump like a small dirt mound and slipped the shit outta the belt when it decided to hit that rev limiter. i told the owner whoops, dont forget to hold that button down when you are backing up in the mud or you'll torch that belt.