Spurious activation of Reverse Thrust
It looks like a spurious signal from the EEC drove the left Thrust Lever to idle to allow for an inadvertent deployment of Reverse Thrust (ala Lauda Air over Thailand). As Auto Thrust was engaged, the right engine will drive forward or remain in position to compensate for the loss of thrust from the left engine. It is this this assymetric condition that brought about the uncommanded bank resulting in the Auto Pilot tripping.