Shawn Michaels is called the Heartbreak Kid (HBK, because when he set out on his own from the Rockers, he was managed by “Miss Sherri.” She gave him the nick-name, “Boy Toy.” That was his first choice for his intro music. He then proceeded to grow with her support until he just outgrew Miss Sherri. Thus, the break-up didn’t set well with Miss Sherri and she was heartbroken. So from that experience came the name, Heartbreak KID. As he grew to be very popularity the fans just began to love him. The performances he gave were second to none. He would literally stop the show at his match. Bringing the soldout audiences to their feet with approval for his talent.

Credit:https://www.answers.com/Q/Why_is_Shawn_Michaels_called_the_heart_break_kid_and_show_stopper