whats so special about iron that causes stars to explode? and when the star collapses, why does it blow up?

Iron does not cause stars to explode.

The reason iron is mentioned is that, a dense star can fuse elements all the way up to iron.

Depending on how dense it is, it can go:

H --> He --> C --> Ne --> O --> Si --> Fe

It will not go past iron because iron has the highest nuclear binding energy of all elements. (actually, 2 isotopes of iron [Fe-58 & Fe-56] ; and one of nickel [Ni-62])

That means, these elements are the most stable elements possible in a fusion reaction.

It has to do with the arrangements of protons, neutrons, and electrons, and the strength of their electrostatic attractions due to their proximity and orientation to one another in and around the nucleus.

Smaller atoms are not held together as strongly, and larger atoms are too big and bulky and unstable to be as tight.

So, because all other atoms have a lesser binding energy than iron, no two iron atoms can fuse into anything higher without inputting more energy than is release. That means the process absorbs energy instead of releasing it, and it will only happen when there is a huge amount of energy such as a supernova. Only then do we get elements higher than iron.

But the supernova is not caused by iron. There are several types of supernova, but the type II may occur on a star with an iron core, and in that process, because there is not enough energy to fuse the inert core, the gravitational energy causes the matter to degenerate (a.k.a a core collapse), and as the center of the core implodes into a neutron star, there is a rebound effect which blows off the outer material.