Quality table tops are built from many smaller pieces of wood, which are then glued together. (Here we focus on table tops, not on butcher blocks, where multiple long beams are glued, then cut into successive slices.)
The pattern on the table top easily identifies how it was assembled.
The advantage of assembly in this way is to reduce the effect of shrinkage or expansion of any one piece of wood. Even if one warps, its effect will be small, and there is a good chance it will be held in place by the adjacent pieces. If the stress does cause trouble, the difficulty will be limited to the development of a crack on one side of the table, and an additional bit of glue may well solve the problem by maintaining the integrity of the table top.
Despite all this, a table top made as shown in the pattern above can eventually warp.
What causes this cupping? The many pieces of wood have now cupped as a single element, and so the stress from any one of them is not responsible.
One thought is the following: If a table top has cupped and now looks like the sketch above, then that means that the bottom side has absorbed more humidity than the top side, because wood expands when the space between the cells absorbs humidity.
Is this correct? Sag is of course potentially another cause, but we assume here that the table top was resting on a rectangular frame underneath that reached to nearly cover all four sides.
We're not even talking yet about how to fix the problem, merely about the effect of humidity on such a table top.