Why, how? Well, imagine going to a wiki article and not just reading the article but clicking every listed link and footnote within the article (at least the ones that interest you) because you want to know all about a given topic. Since you're gonna get back to those after reading all the main article, you just have those tabs open. Now you finish and go to the next wiki and do the same thing. At some point, you've gotta check VN and a couple of other sites you visit everyday and at VN you pick up 5 more articles you intend to read. Now someone else sends you something to look at and then you're interrupted. It can get out of hand quickly.
This last time, was more of a one off. 100 tabs I might be guilty of stockpiling but the number I had accrued this time was insane even for me.