G-Canon

No official explanation has ever been given as to exactly why Papaltine exploded.

The script provides no insight:

The Emperor's body spins helplessly into the void, arcing as it falls into the abyss. Finally, when the body is far down the shaft, it explodes, creating a rush of air through the room.

C-Canon

This was an example of Dark Side Burst, which Wookieepedia describes as

A burst of dark side energy occasionally occurred when a powerful Sith or Dark Jedi was killed. The resulting shockwave was sometimes very powerful, other times it was a simple dissolving of physical matter.

Unfortunately, not even C-Canon has ever really explained just what causes the Dark Side Burst.

G-Canon

No official explanation has ever been given as to exactly why Palpatine exploded.

The script provides no insight:

The Emperor's body spins helplessly into the void, arcing as it falls into the abyss. Finally, when the body is far down the shaft, it explodes, creating a rush of air through the room.

C-Canon

This was an example of Dark Side Burst, which Wookieepedia describes as

A burst of dark side energy occasionally occurred when a powerful Sith or Dark Jedi was killed. The resulting shockwave was sometimes very powerful, other times it was a simple dissolving of physical matter.

Unfortunately, not even C-Canon has ever really explained just what causes the Dark Side Burst.