In most cases it's to help run mounting hardware or duct work. In road racing we sometimes move the caliper mount to the back to allow the cooling brake duct to be routed at the front of the rotor and direct the opening to the center of the rotor. on formula cars, it helps the aerodynamics. It's mostly a clearance hardware mounting issue related to all the other things around the rotor: shock upright mounts, swaybar mounts, A arm pickup points etc. It is unfortunately not to balance things out as was suggested above.
Well when you're talking bikes (especially the motorcross bikes that fly 40 feet in the air) you're only primarily concerned with weight, at least according to the Husqvarna people I know. Also, BMW, a company who has become obsesed with perfect weight distribution seems to mount calipers back-ass-backwards... but then again BMW is one of the few companies who sell sedans with factory brake coolers.