What I find interesting is that at least in the patent of the TCMA, the TCMA copies running on channels A and channel B of the FADEC can trigger the fuel cut-off independently.
Do we know how the throttle position input looks like? There must be at least two sensors (potentiometers? resolvers?) per throttle lever. Do channels A and B of the FADEC receive both (or all) throttle inputs and validate them? Or is a sensor fault expected to be handled by the two FADEC channels disagreeing?
If for some reason each channel gets only one input, or TCMA is a separate process and does not use the validated throttle position but rather one of the raw inputs, then a single throttle sensor fault could trigger the fuel cutoff. It is still very unlikely that it happens to both engines simultaneously, unless indeed there is some short-circuit.
If the TCMA only activates if the throttle is at idle, it further requires that one of the throttle sensors shorts to what would be the idle position. If the rest of the FADEC detects a disagreement, the final command to the engine might not be idle (but e.g. keep current thrust), so at some point the TCMA will trigger as you hit the ceiling of the falling thrust contour.