Why did Tommy John need ulnar collateral ligament surgery?

Maybe everybody knows the answer but how did Tommy John (born May 22, 1943) hurt his arm so badly that he needed revolutionary surgery?  I checked his game logs and he did not seem to be overworked.  He was not a power pitcher even before he stopped pitching July 17, 1974 on four days rest after two innings in LA against Montreal; that made 153 innings for the 1974 season.

Starts per days of rest in 1974:
3 days: 5
4 days: 14
5 days: 2

Before that John topped out with 269 innings in 1970 with the White Sox.  218 innings in 1973 when he led the National League in winning "percentage": .696 (16-7).

Tommy John August 8, 2008
by TiMike via Wikimedia Commons

http://en.wikipedia.org/wiki/Tommy_John

John was a soft throwing sinkerball pitcher whose technique resulted in batters hitting numerous ground balls and induced double plays. In the middle of an excellent 1974 season, John had a 13-3 record as the Dodgers were en route to their first National League pennant in eight years, before he permanently damaged the ulnar collateral ligament in his pitching arm, leading to a revolutionary surgical operation.. now known as Tommy John surgery
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There's a bunch of stuff written about all the current pitchers getting Tommy John surgery and theories about why.  Two explanations seem to be prevalent:
- they throw too hard
- they don't get enough rest because they throw too much of the year.

But did any of that pertain to Tommy John?

Returning to the original meaning of the term pitching may be the only solution: throw underhand as was originally mandated.