It was all a Jewish plot. Jewlicious has the specifics. A highlight:

Abbie Hoffman, who was the head of the Yippies along with Jerry Rubin, Paul Krassner and Woodstock's Ed Sanders shook down the concert organizers for $10K, and then exhorted the attendees not to pay the admission price. The head of security was Arthur Schubert, a waiter from the Concord Hotel. Anyone who can handle hungry Jews could handle security of half a million stoned young adults. Speaking of food for half a million... members of the Monticello Jewish Community Center made sandwiches with 200 loaves of bread, 40 pounds of cold cuts and two gallons of pickles.