No telling. The game evolved by incident, so obviously the set position had to do with a particularly devious pitcher. Most -- not all -- such rules evolved in the 1800's, some before the first league.

As Gene H stated, the game developed and evolved in the 1800s and the basis for many of the modern rules go back to the early days of the game. Major League Baseball (the official keepers of the "official rules" of the game) is notorious for not updating the rules on any sort of regular basis. The two legal pitching positions (the set and the windup) were first defined in the official baseball rules in 1950. They existed long before that and there is no way to determine who developed or first used either of the modern deliveries. In 1845 the distance from the pitcher's area was 45'. That was changed in 1879 to 50' and finally to 60' 6" in 1893. Pitchers first delivered the ball under hand. An overhand or sidearm delivery resulted in a "foul balk" being called and three foul balks in one inning resulted in a forfeit. Pitchers originally pitched from a 12' x 4' "pitcher's box" and could get a running start prior to delivering the pitch. The size of the pitcher's box changed several times until 1893 when it was eliminated. In 1893 the the modern concept of a pitching rubber (plate) was adopted. With the adoption of a pitcher's plate, the ability to get a running or walking start was eliminated and the wording "one step back and one step forward" was introduced for the windup delivery.