Speculation is conversation...
Strong turbulence reported by PAX just before accident. I vote for the wake turbulence issue (757 is a heavy dude in this matter in spite its real weight). The strong and abrupt shear of such encounter, not only causes loss of control but also can cause the jet engines to stall and flame out, if this was the case.
Differently from Airbus, in a 737 you can always override a malfunctioned autopilot, so I don't believe that was the cause.
Well, 737 had a few terrible accidents in the past caused by sudden and uncommanded rudder movements. The NG, however is believed to be trouble free in this matter, so far (Rudder Pressure Reducer installed).