That's just what wood does (as part of "seasonal wood movement"), due to the differences between tangential and radial shrinkage ratios. It can be compensated for by how a slab is cut at the sawmill, or through construction techniques.

But with a big slab cut from anywhere except dead center down the tree trunk, it will not stay totally flat from season to season. Bracing it with a frame structure to keep it more flat is one solution, though in the long run it may crack/split if prevented from warping. Sealing it with a humidity-blocking finish is another approach, but difficult to achieve in practice.

An interesting tidbit I have learned from the biz, is that platters (cheeseboards, breadboards, trophy mounts, etc..) can be sliced diagonally from a whole trunk segment, in such a way that the tangential and radial shrinkage ends up dead even. So the small platters don't warp (much). Just gotta pick the right angle of cut for the wood species in question.

Cheers