There is a finite supply of A titles available. Most collectors like to have some of these titles as historically they are the best games. There is also the realization that they are not depreciating like most other toys, in fact that have been appreciating.
More people hold onto their A titles and thus supply dwindles, prices increase.
Not sure about the future - it really depends how good the next batch of JJP/Stern titles are. If they become top 10 titles then older A titles may drop in value. Stern have only really made 3 top 20 games in a decade, LOTR, SM, TSPP ( Tron the jury is still out ).
Will be interesting to see how WOZ turns out and what the second pin is going to be....