As explained by the script's creator, Amit Agarwal at Digital Inspiration, explains, they're called egg followers because the default Twitter account image is an egg with a colored background. In fact, that's how the script searches for them. It looks at the URL of each of your followers Twitter avatar and pulls any with the word default in there. Of course, not every egg account is a bot or fake. Some folks just like to remain anonymous. In that case, you can see their recent activity and whether they've ever tweeted at all.