Elevators and buyers pay by weight (a bushel volume) based on 16.5% moisture.  This moisture allows the kernel to be stable (won't germinate, typically will not mold) but flexible enough to handle without breaking.  A farmer is "docked" for moisture above 16.5%, the elevator is buying excess water.

Corn kernels naturally lose moisture when mature, when all the starch is deposited.  Warm (70's F), dry and especially windy weather dries grain much faster than say 45 F with intermittant showers every few days.

As a side note.  Not all volume of corn of the same moisture will have the same weight.  The starch can be deposited very densly or not as dense in the same size kernel.  This is called "test weight" and there are premiums or dockage for test weight as well as moisture when sold.  Often high moisture corn will have low test weight as well so a farmer might get docked twice on sale of that grain.

Some years farmers are harvesting corn over 20% moisture. This happens because they have contracts to deliver, it was a cool fall delaying maturity, it has been raining, and soon it will freeze.  High moisture corn that freezes often losses quality through cracking, breaking and is more likely to mold, and usually has very low test weight.

Farmers can sell their high moisture corn to an elevator but will be docked for all the moisture and quality issues.  Alternatively they could harvest high moisture grain, run it through their own grain bin, with heat and air, to dry to 16.5% and then deliver to an elevator.  Typically large farm operations have high capacity driers and can dry cheaper than than the elevator.  Cheaper in this case is the cost of NG vs the reduction in price per bushel an elevator would pay for high moisture.

Farmers don't want to sell corn much below 16.5% moisture because they get paid by weight.  Over drying or waiting too long to harvest costs money at the elevator because they will be delivering less total weight per acre than if it was 16.5% moisture.  Elevators also don't like super dry grain, say below 14% because it breaks and makes too much dust which is explosive.  So farmers (you guessed it) get docked on price if they are too low or too high in moisture.

Farmers carefully monitor their fields and when moisture is at or near 16.5% they harvest that field.  Farmers always plant multiple varieties that mature at slightly different times based on growing conditions.  When a field (or variety) is ready they harvest as fast as possible and deliver to an elevator.  We are talking hundreds of acres per day per combine.

Combines, tractors and 80,000 pound tractor trailers all use huge quantities of diesel to harvest and move this grain.  But the real energy hog in the fall is typically drying costs.  At some point farmers can't wait anymore for mother nature to dry the grain.  They have to do it artificially or they start losing money.  This year was very unique in that almost the entire crop was harvested without need of artificial drying.

Farmers could wait for all their corn to be close to optimum price for sale (based on % moisture) before combining.  This was doubly important this year because it was the second largest corn crop ever harvested in the U.S..  A lot of NG didn't get used this year that usually would have.  So thank mother nature (at least in part) for keeping NG stocks high until now.