Could the pinion have been set to close from the factory?
It could be that or if you've landed on the rear end from a jump. I did that and it ate my spur right up the next time I gassed it. T-bone racing makes a bumper for the rear (and front) the helps keep that from happening.
I have never had the pleasure of doing extreme jumps unless you considerl 6" curbs extreme. I might have backed into a curb or wall in reverse. I do have the T-bone bumpers and I guess they help in most cases but it some cases I think they protect an area while compromising another area. Case in point is I have never broken anything without the bumbers installed. With the bumpers I cracked a bulkhead on one of my firestorms and a bulkhead and top brace on another.
In any event I wrote to HPI and explained what happened. They indicated that it could have been set to tight at the factory and are sending me another spur. Not that it cost that much, but it's nice of them to make the effort. I just hope the spur is the weakest link in the drive and that it didn't cause other problems. I think it will be alright. The car runs fine and I might even be able to use the old spur as a backup if they are reverseable or if I adust the pinion since it is only worn on the front half of the spur.