Its probably because there is no airflow to keep the VTX cool. If its possible to unplug your VTX while working on the bench, that will be ideal. Otherwise you may want to turn the VTX transmittion power to as low as possible, (or enable pit mode if its supported) this way the VTX will stay cooler for longer on the bench. Failing all that I would sugest putting a fan, or some other cool airflow over your drone while you are configuring it to keep the parts cool!.