A throttle return spring sure does help with dead batteries! When the batteries die or get shaken loose or whatever, its the only thing that would help. When you have no power, neither do the servos, allowing a spring to close the throttle if its set up properly.

It sounds to me like you had some localized interferance that may have created a full throttle condition, the failsafe may have helped with that, maybe not.

By the way, do you want to sell me those tires/wheels?