It is hard to say without knowing more about your model/brand of THC
A rough guess is the wires to the voltage divider are not connected correctly or disconnected. Maybe the divider is bad.
When the THc senses 0 volts it will try to raise the torch to correct the voltage to the target set point.
That sounds like what you are seeing.
Can your system display 'Actual Voltage'?
Look at it or measure it going into the THC. (Be careful of a possible high voltage hazard).