During TESB when luke thows the grenade inside the AT-AT why does the "head" explode instead of the middle section

I've always thought that the grenade thrown in by Luke causes a sort of chain reaction. The grenade blows up causing more explosions in the middle section of the AT-AT as bits inside it start to blow that eventually leads to whole thing blowing up and the head going up as well.

Chain reaction--that's what I thought, too.
...But it also just looks really cool for its "head" to blow up.

Hay people(not been on for time)this is my idea.If you known the bits of an A.T.AT most of the main body is open space full of troops, with atmosphere intakes located on top which allows a big pocket of air(stay with me on this) and at the back end is the main fule cells,when luke throws in the bomb which blows the cells and like a shot gun effect with the strengh of the body armor which blows the head

All of the explosions in the original saga are a little overdone. When a vehicle, ship, building, things like that, take damage, all parts of them explode. Like jabba's sail barge. It's pretty big, but one or two shots from its own laser on the deck made it explode entirely.
Only ships of the main characters don't explode after one shot.