I guess I don’t have any idea what universe Citeseer’s collection actually covers, or attempts to cover. I think it’s papers in Computer Science. I think it attempts to be the entire universe of such papers.
You can down load a dump of their catalog in XML. The copy I got has 717,172 publication ids in it. Many of those have a publication data recorded; and most of those are plotted above.
Looks like the exponential growth in these publications came to an end in the 1990s.
Theories anybody? Extra points for amusing theories. For example they all decided to post at Wikipedia and their blogs instead. Or the opportunities in the web space highly distracting, but made lousy papers. Maybe Microsoft hired everybody and drew a curtain around their work.