Here’s the deal: we really don’t know why cats bite their tails in genuine anger and not in play. Cats do play with their tails as if they are toys and as if they are not part of their anatomy. It is strange. However, there is a stranger condition about which there’s a range of theories as to its cause.
It may be that your cat has a little-known condition which still mystifies the experts called Feline Hyperesthesia Syndrome (FHS). If we assume for a moment that it might be FHS there are three theories as to the cause of this condition (1) it’s a behavioural disorder caused by stress or “behavioural displacement”1 (2) a seizure disorder or (3) a syndrome which is caused by a mixture of circumstances including the environment and behavioural issues. I have added two of my own below.
When you search for causes of FHS you do not find clarity. The best book on home veterinary care that I have does not even try and refer to the causes of it. They simply say that the treatment might include anti-seizure medications, steroids or medications that modify behaviour. Those treatments are based on the presupposition that the causes are mental health issues of some sort.
Pain and discomfort
You might take a commonsense viewpoint as I will in this next paragraph. If a cat is licking and biting a part of their anatomy it indicates that the cat is feeling discomfort such as itching or pain in that area of their anatomy. This leads me very tentatively to the suggestion that a cat biting their tail may have suffered an injury in the spine as the tail is attached to the spine. Specifically, the cat might be feeling pain in the tail or a tingling sensation perhaps due to distal neuropathy (nerve damage at the end of the tail). I am not a veterinarian and I’m just speculating wildly. I feel that I’m entitled to because the reference books are blank. Everybody speculates on this one. But I’d check out spinal injury as one possibility. I think the common sense answer is often the best one. To be clear: I’m referring to the cat feeling pain or discomfort in the tail. They bite it to alleviate the discomfort temporarily. The trick is to find the reason behind this localised discomfort.
Other possible causes
Other possible causes are a genetic inheritance, skin problems, neurological problems (injury?), sensory processing issues and as mentioned above stress. It may be an extension of OCD, for example.
Finally, if it is FHS there may be skin rippling which Jackson Galaxy refers to as “back lightning” and/or muscle spasms throughout the body. A cat suffering from the condition might suddenly turn and attack despite being a loving domestic cat. Galaxy says that they may be experiencing hallucinations and they are very sensitive to petting.
Tails treated as toys
Another possibility has occurred to me while writing this. Domestic cats play with their tails as if they don’t belong to them. They can treat their tails as a toy. Snow leopards sometimes hold their tails in their mouths. If domestic cats can disassociate themselves from ownership of their tail, they may be attacking it because, not as an act of self-mutilation, but as an act of deep mental upset, perhaps stress or an inability to express natural drives. It points to a profound mental health issue. This proposal would be supported by the lack of any scientific analysis of FSH because it is impossible to evaluate a cat psychiatrically. This is a massive weakness in our relationship with domestic cats. It is why vets prescribe general purpose psychiatric drugs sometimes in a hit and miss way.
1. Behavioural displacement: the cat “experiences an overwhelming impulse to complete two or more unrelated behaviours simultaneously…” (Wikipedia authors).