Could you convert the tables into graphs, please? It's much harder to see trends in lists of numbers.

Another possible hypothesis could be satiation. When I first read wikipedia, it dragged me into hours long recursive article reading. Over time I've read more and more of the articles I find interesting, so any given article links to fewer unread interesting articles. Wikipedia has essentially developed a herd immunity against me. Maybe that pattern holds over the population, with the dwindling infectiousness overcoming new readers?

On second thought, I'm not sure that works at all. I guess you could check the historical probability of following to another article.