I don't know the specifics of how they did it, and this style of hacking is outside my skill set, but if a computer has any data port (e.g., USB) open and the box wasn't set-up by a systems/security expert which I doubt 1% of public computers are, you're plugging in what amounts to another computer and unplugging the target is an easy way to get it to reboot with a little "help." This is why you'll sometimes see servers / desktops locked with their ports covered or even filled in with glue. It's also why iPods (120Gb in your shirt pocket) and such are banned at some workplaces. There will be terabyte thumbdrives before you know it.

Related: Portable.