Many have pointed out that very few metals actually burn, and none would be good for top-of-the-line robot creation. So why do they burn???
My theory is that the energy that powers all the robots in the form of atomic fusion plasma is being driven over its melt-down temperature and released from its container from external sources (swords or hammers) and creating a chain reaction of varying strengths depending on the initial disruption temperature.
Basically, their fuel catches on fire and incinerates them from inside!