I've been reading the description and operation of Flap systems. WTBs will stop and hold the transmission whenever SFCCs receive some failures.
A runaway failure of an actuator is a situation where the actuator deflects maximally into one of its stops and stays there. Particularly hydraulic actuators can be subject to these: if the servo valve cannot close due to for instance dirt being stuck in the valve, oil flow will cause this type of failure. Electrical actuators commanded by a servo loop can fail in this mode as well.