One of the
quirky rules that makes
baseball both work and hard to understand.
The infield fly rule states that when a batter hits a pop fly on the infield with runners on first and second base with less than two outs, the batter is automatically out.
If not for this rule, some wily fielder would let the ball drop and force the runners out at second and third base. The runners are in a quandry though, for if they did run the infielder could catch the ball and throw to the base from which they came thus executing double or possibly even triple play.
While Snark is technically correct, batter interference for making contact outside the batter's box is rarely called in the big leagues despite its common occurence. The infield fly rule is always enforced.