We're herd animals in a way and follow trends, if others want something and are willing to pay obscene gobs of money for something others start to want it also. How else do you explain the rise in Datsun 240K prices for example? I also think certain cars come back to being in vogue again, right now 80s cars are starting a resurgence in general, I think with TV shows like "Stranger Things" bringing back the 80s synth style music etc.. all that retro stuff comes back and with it icons of the era, as to weather or not these trends last is another question?
I reckon even the Z31 (300zx's) will make a come back soon, we're already seeing 280zx's go that way, early Mazda RX-7s etc..
With regard to the SC 911's I think because all the other early air cooled Porsche's went up in value the next logical way to get in to what the early cars offered was the SC. In the same way that 2 seater S30Z's are rising in value and the 2+2's are starting to uptick also, it's because people want the experience of an early S30Z but maybe cannot afford to get in at the 2 seater entry point.
In comparison to a Porsche I still find the S30Z prices reasonable to be honest. But the cost of some S30Z parts is getting up there lately. I just hope Nissan starts to reproduce some items like washer bottles and similar... paying $900 for 1 is insane.