It funny how 'mysterious' gets used when the involved describe an accident. As several people have speculated, its going to be one of:
a) Deselection of TR before N1 low enough and asymmetric TR stowing
b) No2 TR was INOP and power was put on No2 for some reason, as per above theories
c) PF was using asymmetric thrust to turn and pushed the wrong lever
I'll put money its not a completely uncommanded power-up.