From SI:
http://sportsillustrated.cnn.com/bas.../fan_violence/
Date: Sept. 16, 1940
Setting: Ebbetts Field
Umpire George Magerkurth was famous for his pugnacity. In the minors, he was arrested once for fighting with a player under the stands. Another time, he and a player were suspended for spitting at each other after a disputed call. This time, though, he apparently was minding his own business when a Dodger fan leaped out of the stands, tackled him at home plate and began pummeling him. Although Brooklyn fans especially despised Magerkurth -- his run-ins with Leo Durocher were legendary -- there's evidence this particular incident was simply a diversion so the fan's pickpocket partner could work the stands.
Edit: A little internet research.