Did you try to program this on a clean model location in the TX?
One more thing I would try to rule out the TX is to put the motor control servo back on the throttle channel and the steering servo back on the rudder channel. Then reverse the throttle channel and see if the problem persists with the trigger reversed.
If so, I would then disconnect the motors and try it with only one motor connected at a time and then try the other motor to rule out a problem motor.
Check the wiring and make sure the motor and speed control battery wiring is isolated from the servo /receiver wiring.