Wood naturally expands and contracts across the grain depending on the moisture content - table tops are often fitted with expansion plates to allow for slight movement - sometimes, if the screws are done up too tightly or no allowance has been made for shrinking / expansion then a split will appear in the direction of the grain.
When you "see" to your table top be sure to allow for this natural movement or you might get the same problem again.
Good luck...
Peccavi
July 2010