Complex sentence: In the 2009 season Jenkins returned as a starting defensive end because the Packers had switched from a 4-3 to a 3-4 defense, Jenkins' role switched.
Split into simpler sentences: In the 2009 season Jenkins returned as a starting defensive end for the Packers. Because the Packers had switched from a 4-3 to a 3-4 defense, Jenkins' role switched.