During the AFI Lifetime Achievement Award ceremony for Steven Spielberg, George Lucas said that when Spielberg was told how much time they had to shoot the scene that day, Spielberg said, "The only way we can get it done in that amount of time is if Indy just pulls out his gun and shoots the guy." A bunch of crew members cracked up when they heard that, so they decided to change the scene.
The bit about it being Harrison Ford's spur of the moment improv appears to be an urban legend, given that both Ford and obviously Spielberg were in the audience and they both laughed in agreement with Lucas.