why explosion occure

First you must know that fire needs fuel, oxygen, and heat. When this happens a lot of gases are released from the fuel. If this happens in a closed container the gases build up and store energy. When the container can't hold any more energy this is when an explosion occurs. Basicly an explosion is when a lot of energy is released.
a explosion occurs when a time for a bomb is u or if gas or petrolatum leeks.
In military explosion also occurs with the use of an EFP (explosively formed penetrator)