B777 Captain reported uncommanded autothrottle actuation during taxi. The flight crew returned to the gate for an aircraft swap.

Narrative

On taxi out; traffic ahead stopped; parking brake was set. Once traffic started moving; parking brake was released. Autothrottles engaged uncommanded and moved towards take-off thrust. Manually retarded throttles. Released pressure on throttles and they again moved towards take-off thrust. Manually retarded and disengaged. We then held our position and referenced anomaly section in flight manual additional. Although it addressed this as an 'anomaly'; it was clearly a malfunction and guidance was ambiguous. It seemed to indicate this autothrottles malfunction was due to a short in the system; and could be resolved by manually retarding and disengaging; but no further guidance was given other than it should operate normally on take-off. This seems to be conflicting with normal SOP of getting any known malfunction corrected or deferred by maintenance. We elected to contact Maintenance Control and after discussing; a return to gate was indicated for further analysis. An aircraft swap was accomplished. However; the second aircraft had a similar occurrence on taxi out and subsequent return to gate was accomplished after disconnecting and shutting off the autothrottles system.