I didn't realize you knew Andy Pettitte personally! You can't assume to know the character of someone you only see on TV. You just can't know who would be capable of doing anything. Even if you DO know someone personally, you can still be surprised to learn that a good friend could cheat on his wife or steal from his employer.
After a quick scan of the report, it looks like this Radomski guy was a link to a lot of players. He was with Toronto when Clemens was there and moved to the Yankees at the same time Clemens did.
Todd Hundley allegedly brought it with him from the Mets and got Lo Duca into it, who hooked up Kevin Brown and Eric Gagne. This, by the way, is a plausible explanation for how quickly the Dodgers dumped all of these players as soon as they were able. Hundley tanked right after he came over to LA, but the others might have been thought to be overachieving and on the verge of permanent breakdown.
I noticed that myself; it's very clear there were pockets of players that were on teams together that started the use, and either moved to other teams alone or with trainers who spread the use.
Of course, we don't know how complete this list is. It could be that it only looks so chart-like because there was only a limited amount of people willing to provide evidence, and it pointed back to those in their own circles.