It sure seems like a heck of a lot of web designers are also photographers, doesn't it? And by "photographer", I mean they own a DSLR and heavily consider aesthetics when taking a picture. Why the overlap of interests? Well clearly web design often incorporates photographs. Maybe we like to have the equipment we need to take decent pictures for websites and it grows from there. Maybe it's because photography is a lot like design. You have a canvas of sorts and you need to take into account the principals of design: color, balance, line, shape, contrast... the gestalt.
I think maybe the connection runs a little deeper though, in that it appeals to our inner nerd in the same type of way.
CSS isn't a particularly difficult language to understand. You select things, and change the values of different properties. There is no logic, no math, no object oriented concepts, no security concerns... It largely reads like plain English. Yet, understanding it and being really good at it are pretty far apart. The old adage applies nicely:
It takes a day to learn and a lifetime to master.
CSS might seem a little mystifying at first, until you dig in a little and then you might think "this is all there is?" I found photography to be similar. At first, a DSLR can seem like a maze of settings and details to know about. It seems like learning how to change everything according to the circumstances and your desired outcome is going to be incredibly complicated. Then the further you dig in, you realize it basically boils down to your ISO, shutter speed, and aperture. Then it happens again, "this is all there is?"
It goes deeper in both directions. Yes, CSS can get more complicated with stuff like CSS3 transitions, cross-browser quirks, and trying to style form elements to look decent. Yes, photography can get more complicated with things like different lenses, dealing with white balance, and processing RAW. But those things are somewhat minor compared to the core concepts. Master those fundamentals, and the rest comes along with it over time.