His particular performance at that event was plagued by certain problems that also adversely affected his live vocals.
In terms of technical issues, there was definitely something wrong (Michael Jackson at one point even looks at the mixing desk).
Also, they did not play the instrumental version of the song, instead they chose to play the studio version of the song which seemed to confuse the singer while he was trying to sing live his lines.
Some people noted that this event was a disaster for Michael Jackson, such as the English celebrity publicist Max Clifford who noted that the singer at that event destroyed his on stage image (as a performer) by singing one mangled line and several messy high notes.