TLDR, It could be it because :
- There were other Dark Jedi that exploded when they where killed.
- It's physically impossible for the Emperor to reach the Death Star's core within the time preceding his explosion.
While PhilPursglove's answer reminded me of the death of Joruus C'Baoth, I don't think it's related to Force Scream. His use of that power happened before his death.
At this time, Leia Organa Solo, Han Solo, and Talon Karrde entered the throne room. C'baoth unleashed a Force Scream, stunning everyone in the room. In a rage, he began to bring the ceiling down on Mara Jade.
His death happened later :
Mara Jade defiantly refused and charged towards C'baoth, guided mentally by Organa Solo. When she reached the insane Jedi, she cut him down with the lightsaber. He exploded in a blast of dark side energy.
This quote points to Dark side burst article which fit perfectly the phenomenon. The Mara Jade Quote at the top of this article is also quite on topic :
Too bad we don't have a Dark Jedi handy we could kill. Remember that big blast when C'baoth died?
-Mara Jade in Vision of the Future.
As for the Emperor, his explosion cannot be caused by the Death Star II's main reactor, since it's size is evaluated at 900km. his fall preceding his explosion was a mater of a handful of seconds, falling to the center of it would imply a one hour trip at the improbable mean speed of 450km/h.
Another theory is that he could have hit something at the base of the tower in which the Throne Room sits.
The Emperor's throne room on the second Death Star was on the top of a 100-story tower located on the north pole of the superweapon.
A 100 story Tower makes it around the height of Empire State Building, so something in the vicinity of 400 meters, which better fit the duration of his fall preceding his explosion. But I found some Schematics that confirm that the tower sits on a "Power distribution Shaft". So, he probably hit something, but it most be something else than a reactor core. It was what killed him, triggering his explosion.