Short story: Carbuncle fights you because the Celestial Avatars are total dicks.
Longer story: When you go to the Full Moon Fountain with the enhanced power from the harder Celestial fights, the Celestials show up and try to take all of their power back. They were being channeled through the player, so Carbuncle says that they only way to stop them (and the potentially disastrous consequences) is to defeat the one who is summoning them. (i.e. you) So, they were being total dicks. But the good news is that the Celestials didn't seem to totally hate humans in the cutscene, so maybe Carbuncle's plan is working.