Philly.com (paywalled):
Pa. gerrymandering's surprise co-conspirators: Democrats
This map we've been choking on? In 2011, one in three Democrats voted in favor of it. Without their votes, it couldn't have been the final map. Why'd they do it... philly.com says, local interests prevailed. The state Ds were disorganized, so local Ds were happy to go along with the plan if it protected their own district.