I suspect it was because Next is right next to Waterstones, but that really doesn't matter. One of the dummies in the window was wearing a T-shirt I just had to have.
Anyone who's had their hands inside a car engine will probably have used a Haynes manual to guide their first fumbling tweak with a shiny new socket set. So when I saw a shirt that purported to be the cover of a Haynes manual for an Imperial AT-AT walker...
Three of my geekinesses in one item of clothing? What's to say no? And there was one in my size!
Now if only I had a socket set big enough.