The outro riff is a recap of the main thrash riff.
One of my favorite things to do when I'm composing is to bring back ideas that were previously heard in the song. Keep it thematic people! :) Sometimes even if it's just a small chunk of a riff it can have a great nostalgic effect. It sometimes take a keen listener to notice these kind of things but this is a very simple and obvious way that I've in incorporated this idea.