This is a really fun question because it is not very clear why some chemicals can explode, but if we think about some basic rules for how chemical reactions work, we should be able to find an answer. The first thing to note is that chemicals will not explode on their own, they need to undergo a reaction first This could be as complicated as adding multiple molecules together or as simple as exposing the chemical to the air. When molecules react, the starting chemicals (called reactants) have a certain structure to them. The atoms that make up the molecules are connected by bonds, and during a chemical reaction, bonds can be broken or created to form the products (the ending chemicals).

Depending on the number of bonds that are either created or destroyed, the reaction will either release or absorb energy. A reaction that absorbs energy will feel cold because it is taking heat from the environment to use as energy to fuel the reaction. This is called an endothermic reaction. If the reaction releases energy (called an exothermic reaction) you may notice that the reaction heats up. If that energy is released very quickly and violently then the reaction can cause an explosion. Thank you for your question!

Chemical reactions release heat. Lots of heat in a small volume makes lots of pressure. That pressure then expands so rapidly that it breaks the sound barrier, i.e. creates a shockwave. This is what we call an explosion.