A lot of times, its the ump! :shock:
Doing anything well when under pressure is a combination of mental, physical (or physiological) and emotional preparation...and being a pitcher is about as pressure-filled as it gets in sports IMHO
In order to throw strikes, you have to have a certain physical dexterity/ability. If you have the knack of throwing the ball accurately, you then have to work on doing it well in pressure situations, at full exertion, with some gorilla 60 feet away looking to swat the next one back to Africa. That takes more than physical ability; it takes lots of repetition before game day - in the off season, too; it takes confidence and focus both before and during the game. (It helps the pitcher to throw strikes confidently if he has a solid defense behind him, too).
But some guys are just born with "it". From the very first time they pick up a ball they throw hard and accurate. They throw strikes because thats what they do...and they may never understand how they do it, or why everyone else can't. They go to the mound with the confidence of ignorance...not a bad thing in pitching or in marriage...and let fly. These folks are born with the "magic bullet" already loaded in their gun.