Anything that reacts and releases energy can explode if the energy released is fast enough. The explosion is caused by the energy being unable to be contained in the small space of the chemicals. The more reactive the chemicals, and the smaller the space in which they are confined, the easier it will be to have an explosion.

Here's an example for you: think about gunpowder. If you just have a pile of gunpowder and light it, it will burn - it will not explode. This is because the hot gasses produced by the burning can go into the air without having to force their way out. Thus, no explosion.

Now, think about a firecracker: you have gunpowder wrapped up in a small tube of paper that holds the gasses in. Light the gunpowder now, and it will have to burst out of the wrapper. This creates a small shockwave - the loud "crack" that you hear - in other words, an explosion.