Volume risk is a commodity risk which refers to the fact that a player in the commodity market has uncertain quantities of consumption or sourcing, i.e. production of the respective commodity. Examples of other circumstances which can cause large deviations from a volume forecast are weather (e.g. temperature-changes for gas consumption), the plant-availability, the collective customer outrage, but also regulatory interventions.
Example
A electricity retailer cannot accurately predict the demand of all house holds for a given time which is why the producer cannot forecast the precise time that a power plant will provide more electricity that consumed, even if the plant always delivers the same output of energy.
References
Volume risk Wikipedia(Text) CC BY-SA