They are spin stabilized, so no, I don't think that yaw would occur until the control systems run out of fuel.
I know... but some of these spacecraft that are displaying this anomaly are far beyond their expected and designed life and probably have long ago exhausted the stabilizing fuel (it would be too expensive in Delta-V to provide a fuel supply that would exceed mission parameters, don't you think?) yet they are still sufficiently oriented toward Earth that we can receive their signal. Also, I think that the control people would have noticed an unusually high number of stabilizing maneuvers as the vehicle re-oriented itself if that were the case. It appears to me that whatever is applying a force to the vehicle is acting on the vehicle as a whole and not preferentially on any of its parts due to size or positional leverage.