Why does an implosion or explosion occur?
I look at it from a gas point of view. An explosion results when a quantity of material (the explosive) is converted to gas at a high pressure and temperature, as the result of a chemical reaction. The hot gas expands rapidly, creating the explosion. An implosion is the opposite. A volume is suddenly changed into a vacuum or a lower pressure than the surroundings. The surrounding air or whatever pushes in and causes the material to rush into the lower pressure volume. This rushing in material meets in the center and the speed of the movement creates a temporary high pressure and temperature area, which then explodes. Two examples of implosions are a large CRT (vacuum inside) that breaks, or a submarine that suffers a major leak.
