it cooks things from the inside first so i suppose it causes an explosion but your asking the wrong person for a science explanation ask ethel shes clever i bet shes got an ology and everything lol
The microwave cause an electric current to run through the tin foil, which due to the foil being so thin causes it to heat up vary fast which is enough to be a fire risk in it's self. on top of that any pointy bits of the foil (if it's crumpled up for example) can cause sparks to ark across to the walls of the microwave, also a fire risk.
Though it's actually highly unlikely that it will cause any damage to the microwave itself (well unless it catches fire)
No it doesn't explode.
Metal reflects the microwaves back around the microwave container. With earlier microwaves there was a danger of damage to the cavity magnetron - the microwave generating gadget in there.
Several pieces of metal in there will become charged with very high voltage but very low current. Under certain conditions, arching may occur between the different pieces of metal, or between them and the case.
It just isn't worth experimenting by trying it.