The Dodgers played brilliantly in July, going 21-7 and finishing the month with a three-game lead over the second-place Padres. Why would you mess with the roster that did that? Because the Dodgers did not, at that point, appear to have good enough starting pitching to make a splash in October. But to get Brad Penny (among others), the Dodgers gave up Guillermo Mota and Paul Lo Duca.
The Dodgers haven't played as well since the trade as they played just before the trade, and the most popular explanation is that Eric Gagne has struggled. It's certainly true that Gagne hasn't pitched nearly as well in August as we expect him to pitch. In July, he was phenomenal: 15 innings, seven hits, two runs, one walk ... and 28 strikeouts. In August, he's been decidedly non-Gagne: 15 innings (again), 17 hits, eight runs, five walks, 17 strikeouts ... and three losses, after losing only four games in 2002, 2003 and the first four months of 2004 combined. So yeah, he's going through a rough patch.
But are Gagne's struggles related to the big trade? That's a real stretch.