its just popped into my head but did you install the servo yourself? did you set up the servo to neutral position before you placed the servo arm on for the rudder connection. i have seen inexperienced people to this before they just shove the arm on with out setting the servo to neutral, what tends to happen if they just shove the arm on with out alignment of the servo, the servo gains power and resets to neutral but due to the arms position its unable to turn locking up the servo and after a short amount of time burring it out or breaking the gears.
not sure if that the issue but its one of them, if thats the case hope you didn't bust the servo, if your lucky remove the servo arm. turn the power on and let the servo reset to 0 and adjust the TX Trim to the middle or neutral. then put the arm on so the rudder is as central as possible then screw the arm in place so not to slip later.
this is proably not the issue but it could happen,
i have had similar experience doing that, my issue was using an old mechanical esc i forgot to reset the trim on the TX to 0 placed the servo arm on. everything was fine .. took it out put the trim to neutral and couldn't understand why it whooshed off without any control... i was so pre-occipied trying to control it i didn't think to pull the trim back on the throttle..
we all have moments like this so don't laugh!