A year sounds about right for this defect to manifest itself. It is a compound problem that may or may not be completely resolved; Hickory is a very uncooperative wood when it comes to stability. "1X6" boards you say, well these are too wide for an average depth like a counter tops- they should have be ripped to 3" wide max, and growth rings alternated. The fact they have shrunk in width so as to leave "grout size" gaps possibly means they were not fully kiln dried to 6% moisture content before construction. The underside of the top should also be sealed with at least as many coats as the top for equilibrium reasons. Did you do this installation yourself or was it subcontracted? If it was subbed, it is a warrantable problem- let him deal with it.
The most likely problem affecting this is something you may be able to cure. Wood moves constantly across its width as the season changes. A good design allows for this to occur without destroying it self- in other words your top may be overly fastened down. Remove the hold down screws and enlarge the holes; use washers when re- screwing. Break any glue joints between the bottom of the top and the top of the cabinet base- you want the top to float, but not slide off. Give this approach some time to work, and those gaps may well close themselves back up. Failing that, I would use colored shellac stick filler and a heated knife to fill the cracks.
"Safety... Always" Hank Metz Producer, "Biscuit Joinery Techniques" video