I think that the answer could be: Newton's second law and an ill-placed girder.
But really? We never saw it happen... so he could have been falling for a long time until the Death Star eventually blew up.
Akwat Kbrana said:
You're all wrong. Emperor Palpatine was killed by global warming.
Inconvenient? The truth often is.
Precisely. The globe in this case being the Death Star, and the warming being from when the reactor erupted.