Answer by taking a quote from the following article:

Johnson was born in Walnut Creek, California, to Carol Hannah and Rollen Charles "Bud" Johnson. By the time he entered Livermore High School, he was a star in baseball and basketball. In 1982, as a senior, he struck out 121 batters in 66 innings, and threw a perfect game in his last high school start. He also played on a Bercovich team that assembled top players from throughout California.

Johnson was sidelined throughout much of the 1996 season with a back injury, but he rebounded in 1997 with a 20-4 record, 291 strikeouts, and a 2.28 ERA (his personal best). Between May 1994 and October 1997, Johnson had gone 53-9, including a 16-0 streak that fell one short of the AL record. Johnson had two 19-strikeout starts in 1997, on June 24 and August 8.  Another colorful All-Star Game moment proceeded in the 1997 edition involving former Expos teammate Larry Walker, at that point with the Colorado Rockies. When Johnson had started an interleague game versus the Rockies on June 12, Walker chose not to play, explaining that "I faced Randy one time in spring training and he almost killed me." In the All-Star Game, Walker batted against Johnson, who theatrically threw over his head. Ever adaptable, Walker placed his batting helmet backwards and switched sides in the batters' box to stand right-handed for one pitch. He ended the at bat by drawing a walk. The incident momentarily drew mirth and laughter from players in both dugouts, fans and announcers, and, of course, comparisons to the at bat with Kruk in the 1993 All-Star Game. In spite of garnering a reputation of avoiding Johnson, Walker batted .393 (11 hits in 28 at bats) against him in his career, nearly double the rate of all left-handed batters at .199.  When the 1998 season began, Johnson was upset the Mariners would not offer him a contract extension, given his contract was expiring after the season. Though the Mariners initially wanted to keep Johnson, turning down a trade offer from the Los Angeles Dodgers, they fell out of contention, going 8-20 in June. Minutes before the non-waiver trade deadline, on July 31, the Mariners traded Johnson to the Houston Astros for three minor leaguers, Freddy Garcia, Carlos Guillen, and John Halama.

How did he get the back injury