They are the code names for the projects so that you can refer to them in emails and whatnot without giving away the actual project. Sometimes they follow a progression like the high end GT200 workstation chips being Tesla, then Fermi comes out, next Kepler, all three were very important scientists.
Bulldozer likely came about with Bobcat(the low power variants) since a bulldozer and a bobcat are similar but very different in size.
Everyone has their own naming scheme, and i think in order to understand where they all came from you would have to talk to a lead designer on each project to figure out why they chose that name.