Of course it could be the rear rotors warped. They can warp just like the fronts.

Another possibility would be present if you did not THOROUGHLY clean the hub surface before putting your new rotors in place. I replace my rotors so infrequently, that it is also a good time to clean and pack the wheel bearings. Since I'm taking off the hub in this case, I strip everything apart, give the hub a solvent bath and then take it to the steel brush that's on one side of my bench grinder. I clean it until it shines on the entire mating surface.

If the wheel bearings are not due a packing, then you need to otherwise find a way to totally clean the mating surface on the hub before putting your new rotors in place.

Hope this helps,