hi jaycrow,
These rims that are on this machine, they are the older type split ring rims aren't they? Those had a rather large hole for the stem to stick out of if i remember right.
I've seen this happen before. What's happening is the tire is spinning around the rim, it's not very noticable but it moves just enough to pull on the valve stem area because the tube is trying to move with the tire and the valve stem is being ripped out of the tube, it makes it crack around that hole where it comes out.
Whoever does the tires needs to make sure the tube is the type that has the reinforced valve stem. Then when they put the thing together it might help to put some baby powder inside the tire so the tube can slip. This will cut down on that type failure quite a bit.
Of course the only sure fire way to fix this is to go with solid tires. I know they cost more but in the long run they are better for that application. Just keep the gravel packed and you shouldn't have any issues getting stuck.