Fire in hot air oven
If you are saying that there was an explosion while you attempted to dry/bake plain loose flour in a fan-assisted oven, then I'd say it was a classic dust explosion (flour explosion). It occurs when fine dust of combustible material (the flour) is dispersed in the air (e.g. by air whirling) and then gets ignited by an open flame, a heat element, or even an electrostatic discharge. This can happen at any temperature. Flour explosions are a known workplace hazard in the food industries and grain/flour storage facilities.

Next time turn off fan-assisted heating in your oven. If not possible, use a different oven.