I am a little unclear if your cat is doing this when you are using the toilet or when he is in his litter box. The action he is doing of backing up, raising his tail and then shaking it sounds like he is spraying. Check the surface to see if he has left behind a fine mist of urine. This could be because he is excited or because he is marking his territory. Some cats will also do this naturally in the litter box. If he is spraying, consider neutering him if not done already, be sure you are keeping your litter boxes clean, limit his access to the bathroom toilet, and talk to your veterinarian.
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When going to the bathroom toilet to do my thing, my cat backs up to the toilet and.. 
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