I happen to have a scraper handy for tracking stars over time, and profiling the users who made those stars (https://github.com/minimaxir/get-profile-data-of-repo-starga...). Here is a chart of the daily number of Star events on the big-list-of-naughty-strings repository as of today: http://i.imgur.com/NzzjuKK.png
The paper assumes that popular repositories are popular on their own merit. But as you can see from my chart, there are clear spikes, one when the repository was first released, one a few days afterward, and one in February for literally no reason. The cause of those spikes were Hacker News, /r/programming, and /r/webdev respectively.
Here's another one of my repositories currently at 458 stars (https://github.com/minimaxir/facebook-page-post-scraper) which only got exposure after a Show HN last Thursday: http://i.imgur.com/8qpTrrb.png
There's also a huge selection bias problem with only looking at Top Repositories. While understandable from a scalability standpoint, it's also possible that repositories are popular because of good marketing.