MJ said it was an urge, a feeling that he got when he felt the music... He didn't know why he did it, he just did it.
The first part of this video explains but I don't know about the rest... The person who made the video is trying to make it seem that MJ grabbed his crotch 24/7...
Well in the interview with Oprah he said that it's the music that makes him do that. "You know I'm dancing and I go BAM!" He said that lol. He was quite embarressed when she asked him cause the fans wanted to know too and Michael laughed and looked down aww.
It's his music that makes him do that, the music is inside him.And the making of "Bad" music video , when MJ danced and when they finished the making of it, they watched the video and they were surpised when they saw MJ grab his croch.And they asked MJ why did he do that and he said that it just happened, he just feels the music, it"s inside him.No one can do the way he does it becouse it's inside him.