Neither Tam nor I have seen a cat do this before: under some circumstances, typically if he is annoyed or frustrated, Huck the cat will "Mohawk" the fur along his spine. It stands right up with the hairs at 45 degrees or so, looking like the crest along a dragon's back.
I've seen him -- rarely -- "bottle" his tail, puffing up the fur so it looks three times its usual size; he can puff out his fur, too. But "Mohawking" is a new one. H'mm -- whatever's irking him doesn't rate a full threat display?
(Rannie-Wu, Tam's cat, has picked up another nickname, from the way she eels across the desk and ever-so-delicately into my lap: Almost Two (or Too) Jaguar. Call it semi-Aztec; and Huck's would probably have to be Ten Tiger, his strength being as the strength of ten because his heart is pure. Greedy, but pure.)
3 months ago