I believe BBB and Tron LE are the two titles that created a collector frenzy. People will deny it, but what could be sweeter than buying a pin, playing it, then selling it for double in a year. That's a dream scenario.
As long as there is more demand than supply of a title, the price will climb. The issue now is this: with remakes and so many new pins on the horizon, it's really hard to see a title spike like either of those again. Premium pins have pretty much guaranteed Stern LE's won't spike.