Not sure where you are located, but there is a fairly easy fix for this problem.
Check out Eva Holiman's "The Boardlady" site and get some of the penetrating epoxy she uses on wood finish boards.
The cracking you are seeing is caused by the wood in your board not soaking up the optimum amount of resin when manufactured to fully seal the wood layer.
The wood layer needs to be saturated to a certain level to prevent the wood from "soaking" with water, and for whatever reason the saturation level in your board is a little low.
So, get some penetrating resin, and refinish the bottom of your board.
If you cannot find any penerating epoxy resin, then I think Eva suggests a good 2 part polyurethane floor varnish, well thinned so it will "soak" into the surface of the wood and seal it.
This will stop the cracks.
Hope this helps,