All markets have bubbles, as peterh points out one of the first being the great tulip bubble of 1637.
Market prices are set by buyers and sellers and one has to wonder, who are the buyers bidding these babies up? Are they speculators? ARC, McIntosh and the more esoteric tube-gear makers use Russian or Chinese tubes for the most part, so it isn't them. I'm unaware of any way to measure the growth, if such exists, in the DIY market which would seem to be the likeliest culprit in demand for NOS tubes, but there can't be that many new tube gear builders to justify the NOS price increases. But the vast majority of even tube fanatics want a plug-and-play situation and aren't going to go tube-rolling. Most ham gear these days is mainly solid-state, so it ain't those guys.
The math doesn't make sense, either. A New Sensor Gold Lion KT-88 will give at least 3 years of high-quality service and costs 50 bucks. A NOS and properly tested TS 6550 runs nowadays for about $175-$200. So the antique Tung-Sol would have to run rock-steady for 8-10 years to meet the value of a new GL. I'd like to meet someone who has gotten that length of service out of any power tube in a guitar or hi-fi amp.
I do prefer the sound of NOS Sylvanias and RCAs in the pre-amp and driver holes, and am willing to pay a premium over the Russkie prices. But I am sure the Russians will catch up, if we quit bombing their factories.
Back to the original question one must posit: who's buying all these NOS tubes? Judging by the price curve, the answer has to be speculators -- people who are buying them not because they intend to use them, but because they are betting the price will keep rising and they'll make a fortune.
Of such thinking, bubbles are created. Think 1929. Bernard Baruch's cab-driver told him that summer to load up on such-and-such stock, it was a sure thing. Baruch knew that when the common man was in for a quick buck, it was time to get out, and shorted everything. The rest, as they say, is history. The NOS $5 6SN7 and the $8 6550 aren't far ahead.