There are a lot of reasons, back when I bought the k-x in Spring of 2010 there was still some worry that Pentax would close up shop, so you have to look at the current prices in relation to where things were in late 2009.
Anyway here is my list:
- Introduction of k-x brought lots of new people into / back to the brand
- Rapid acceptance of shooting video with DSLR starting 2008 - 2009 created a demand for high quality all manual lenses that could be adapted (primarily to Canon)
- Introduction of very successful k-5 and then k-30 continued to add people to the brand
- Release of numerous mirrorless cameras from various brands that can use either M42 or K-mount with an adapter again increased demand for high quality manual primes
- Inexpensive adapters to m4/3 cameras from k-mount again increased the demand for old k-mount glass
- A collector market is alive and well which drives the price even more as lenses get bought and put on the shelf
- Continued speculation about a potential Pentax FF has people either holding onto their glass or scrounging up anything they can find either to speculate on or to use
- Lack of a full line of modern lenses means in some cases the only choice is legacy glass, again driving up the price
I don't see the price increases as anything but good. It means more people using Pentax, either natively or adapted. Although, it certainly makes LBA a little more expensive.