Sometimes the keyboards tuning will determine such 1/2 or 1/4 step tuning things as they are difficult to retune quickly and everyone has to tune to em. But I'll bet MJ's vocal requirements were the cause for the 1/2 step detune you guys are talking about.
For the more scientifically minded, tension pitch and mass are related to each other for vibrating strings and vocal cords. This is why they wrap the heavier guage guitar strings, to get more mass on the string to allow fairly even tension across the chorus of strings. For example , pluck a string and then clamp a split BB fishing line sinker on it and heard the pitch go way down. In 1976 MJ's vocal cord were heavy with all the blow running down his throat and , well the song's gotta go down to Bb tonight guys.
peace.