Who triggered the activity?
Anyone can manually save a URL into archive.org
There are also automated crawlers and I assume they capture pages getting a lot of hits.
This is what I found on wikipedia. I am not an expert on these things.
The Wayback Machine is a digital archive of the World Wide Web and other information on the Internet created by the Internet Archive, a nonprofit organization, based in San Francisco, California. It was set up by Brewster Kahle and Bruce Gilliat, and is maintained with content from Alexa Internet. The service enables users to see archived versions of web pages across time, which the archive calls a "three dimensional index."
Since 1996, they have been archiving cached pages of web sites onto their large cluster of Linux nodes. They revisit sites every few weeks or months and archive a new version if the content has changed. The intent is to capture and archive content that otherwise would be lost whenever a site is changed or closed down. Their grand vision is to archive the entire Internet.[3]