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.