I’m sure you have plenty of reasons to mass unfollow people, but don’t forget that Twitter does have its own set of guidelines of automation. Although Twitter says that they don’t have an upper bound on follow/unfollow actions permitted, however they disallow bulk, automated, and aggressive follow/unfollow behavior.
Finally, how to unfollow all on Twitter – The script
So the script is from JamieMason called unfollow.js. What you need to do to get it to work is this:
Keep scrolling to the bottom until all of your followers are loaded.
Let this thing run for a bit and you should be good to go.