Gunradd,
really sorry to see your model go in.
i know everyone's going to have a pet theory as to what happened, but can I ask the following.
1) did you have the transmitter set up to start each engine individually or both at the same time?
2) did you have a gyro on it that had a heading hold facility on the rudder?
its just the behaviour of the flight looks awful like a rather exciting flight I had one time of my small SM A-10....