Rory McIlroy and the Fall of the Mighty

Seth Wenig/Associated Press

FARMINGDALE, N.Y. — When Rory McIlroy finished his second round at Bethpage State Park’s Black Course on Friday, he tried to reconcile the difference between the two double bogeys in his first three holes and the four birdies over his last six.

“I obviously had a very bad start,” he said as he stood outside the scoring tent. “I didn’t particularly feel like I hit any bad shots. Maybe more bad decisions.”

Whatever the explanation, no golfer experienced a wilder swing over 18 holes at the P.G.A. Championship than McIlroy, a two-time winner of the event and the one-time heir apparent to Tiger Woods as the dominant figure in the sport. McIlroy is more of a mystery than…