There is reason to think that Socrates was guilty as charged. He was charged with corrupting the youth of the city and looking at two his two famous students Alcibiades & Critias, it not surprising the city thought this strange penniless pain in the butt homeboy was a corrupting influence. Socrates was also charged with impiety. Maybe his continually pointing out of the moral weakness of others in Athenian society had graded the nerves of the more aristocratic families long enough. We also know that Anytus, one of those who brought suit, had a son was also a student of Socrates, which probably didn't sit well with Anytus.
A good brief summary here:
http://law2.umkc.edu/faculty/projects/ftrials/socrates/socratesaccount.html