When something goes wrong with my gasifier it is usually operator error
If you had reported that the hopper temp also shot up I would think you had run out of wood.
If the post gasifier temps went high, the air flow resistance at the char bed fell and the hopper temp stayed normal I would think you had a fuel bridging problem or after you clean out the gasifier you didn’t add back enough char .
If your condensate happen to flow over into the fire tube it could also make the temps jump high from the steam and loosen the char bed. .