Started by accident. First game I made was Idiot, and got the domain idiot-cardgame.com for it, since I had no plans to create more at that time. When I made Crazy Eights I couldn't really use idiot-cardgame.com for it, so I gave that its own domain. After that it was just a kind of pattern I guess. I also had some vague ideas that maybe I'd get more google juice out of having links from different domains to each other.

I have thought about changing it all to one domain, and setting up some 301 redirects, but I'm just afraid to mess up my google rankings. For the solitaire game I re-use a domain, e.g I have http://www.solitaire-cardgame.com for Klondike solitaire (or classic solitaire), and then I have http://www.solitaire-cardgame.com/freecell for Free Cell solitaire.