I would say it is 99% a mental thing. In BP they are relaxed and just doing their reps. In a game, the pressure is on- getting signs from the coach, the situation, and of course the bad call from the ump on the first pitch can totally affect the batter’s psyche. It doesn’t take much. Even a slight distraction means the difference between fouling it straight back, and launching it over the centerfielder’s head. I think that is very common. Look at all the rookies in the MLB right now making their debut. One that comes to mind and reminds me of my son is the Yankee Ford who just came up. Great plate discipline, but keeps in just missing a big hit. Finally got a double today. Confidence is everything as a batter. If that confidence falters even slightly, it turns a slugger into an irrelevant batter.