This chart shows the average number of citations papers in a given year have garnered since their publication. Again this is the citeseer data.
Let me those offer three very tentative explanations. I’m sure people can come up with more. Presumably, it’s easier to write a fundamental, useful, provocative paper when a field is young. Preferential attachment effects help older papers to bank more citations. Noting how few papers are in the collection in 1985 I suspect the collection manager is seeking out the papers from that era and starting with those that are cited.
Citeseer works by reading the citations from PDF and .PS files. Its biased towards them and not HTML. It probably doesnt pick up much before ’85 as there was no postscript before then.
As to why things tail off after 2000; I dont know. Maybe we’ve all moved to powerpoint instead