Why does the ferrovanadium reaction require only enough heat to start it?

The ferrovanadium reaction requires only enough heat to reach its kindling temperature because the process is exothermic.1

In this reaction, vanadium pentoxide ($V_2{O}_5$) is reduced by aluminum to produce vanadium metal, which then dissolves into molten iron. Because this oxidation-reduction process releases a significant amount of heat, the external heat supply is only necessary to initiate the reaction by reaching the required kindling temperature of 950° C (1,750° F). Once this threshold is met, the reaction becomes self-sustaining, allowing the external heat source (such as electrodes in an electric-arc furnace) to be withdrawn until the process is complete.1