According to 'Seconds from Disaster' the co-pilot was the pilot flying through the whole incident. He was also unaware that on the Bus, while in flight, the rudder becomes very sensitive to pedal movement. In fact, only a couple of inches of pedal movement put the rudder at the full stop. Being unaware of this he used a technique to recover from wake turbulence that he was taught by American Airlines which uses the rudder. However, by unknowingly moving the rudder too far, back and forth, he made the wake turbulence oscillations worse.
Eventually, to my understanding, all of the unusual aircraft movement was being caused by the pilot and his excessive rudder movements. As the aircraft yawed more and more the pilot's thought that they were still stuck in the wake and continued increasing control movement to try and correct. This eventually led to alternating full-stop deflections which caused a massive structural failure in the vertical stabilizer.
Hit up YouTube and search for the flight number, it's a very interesting video.