Well there is a scientific explanation to that. Explosives under certain conditions trigger chain reactions within themselves. These conditions may vary, it may be heat, exposure to a certain other chemical or any thing else! Once the reaction starts it forms a chain and the power keeps on increasing!
Chemically the rate of reaction reaches exponential levels rendering the reaction powerful and explosive. If the raw material used in such reactions will be in proper quantities( as in case of explosives like TNT or others) the reaction can take a devastating spree on its hands!!