I am an engineer and not a physicist. I am unable to understand why SuperNova happen.
I understand that when the core is composed of high atomic number of elements like Iron and further fusion is not possible. The core collapses under its gravity.
At this point the outer layer burst in a huge supernova. I am unable to understand why the outer layer just burst out in this supernova. Leaving behind a Neutron star or a black hole.
You are in good company, the most likely mechanism is called the R-process, relevant extract from article:
Immediately after the severe compression of electrons in a Type II supernova, beta-minus decay is blocked. This is because the high electron density fills all available free electron states up to a Fermi energy which is greater than the energy of nuclear beta decay. However, nuclear capture of those free electrons still occurs, and causes increasing neutronization of matter. This results in an extremely high density of free neutrons which cannot decay, on the order of 1024 neutrons per cm3)[3], and high temperatures. As this re-expands and cools, neutron capture by still-existing heavy nuclei occurs much faster than beta-minus decay. As a consequence, the r-process runs up along the neutron drip line and highly-unstable neutron-rich nuclei are created.
Not relevant to your question, but some interesting practical aspect of the R-process:
It has been suggested that multiple nuclear explosions would make it possible to reach the island of stability, as the affected nuclides (starting with uranium-238 as seed nuclei) would not have time to beta decay all the way to the quickly spontaneously fissioning nuclides at the line of beta stability before absorbing more neutrons in the next explosion, thus providing a chance to reach neutron-rich superheavy nuclides like copernicium-291 and -293 which should have half-lives of centuries or millennia.
Here is a simplified picture of what happens in a Type II.
The star's structure is supported against gravity by the heat being generated in its core by fusion processes. When the star burns all the fuel in its core to iron, fusion stops (the heat source is extinguished) and gravity then takes over and collapses the star.
The core of the star then gets tremendously squeezed down by the huge pressure applied to it by the inrushing mass of all its outer layers. It resists that pressure and rebounds, kicking the inrushing matter back out again with tremendous velocity as a shock wave traveling outwards against the inrushing matter. When the shock wave makes it all the way out to the surface of the star, the outer layers and a lot of the iron core get blasted out into space.
During the crushing of the core, the pressure is great enough to mash electrons onto the protons in the iron nuclei, which releases a gigantic flood of neutrinos. Moments later, those neutrinos emerge from the exploding star and zoom off into space.
Hours later, when the shock wave makes it out, a huge burst of ultraviolet and visible light comes streaming out of the remains of the star.
In the weeks and months that follow, the highly radioactive nickel and cobalt nuclei that were formed out of the iron in the core decay and spewed out into space release radiation, the energy of which is a telltale sign of that decay.
The supernova of 1987 provided us with the early neutrino burst, the ultraviolet burst, and then the nickel-cobalt-iron decay radiation, confirming this picture of the Type II supernova mechanism.
