It's difficult for me to pinpoint what exactly caused my therianthropy, or whether or not it was even a choice, because I don't remember when I started feeling this way. My memory only goes back to about the age of five, and I've been acting feline for as long as I can remember.
I was raised around Bengal cats, and was very close to my first cat, an old girl named Squealer. She was like a grandmother to me almost, she taught me how to have empathy for animals and bond with something not of your own species. She was there for my birth, up until she died when I was about seven. It is very possible that she had imprinted on me at an early age, causing me to develop an attachment not only to cats but to spotted, half-wild, or wild cats in particular. That and childhood trauma from being excessively bullied and isolated during my formative years likely could have contributed to the development of therianthropy, it's just been too long, and my brain blocked out too many of the memories for me to remember accurately. When it comes to things like this, I don't want to guess and end up being totally wrong, as that might send me or somebody else into a complete spiral, wondering if we're valid, if we're real, if something went wrong along the lines that led to our current state, etc. Ultimately, I don't care what caused my therianthropy — I still feel non-human and I likely always will, I view it more as a matter of 'what just is' rather than what could or should have been or
why it is.
I also would like to add on to what @
Jeb_CC said about disassociation: I think disassociation is simply required for some aspects of therianthropy. In order to recognise and accept this part of yourself, you have to willingly detach yourself from humanity, even if only momentarily. Furthermore, when you shift or do something to feel more in tune with your theriotype(s), you are essentially disassociating in order to bring about the animal in you. Disassociation can also be a common way of coping for youth experiencing early or extreme trauma. Somewhere along the lines, amidst trying in desperation to detach yourself from reality, the human and animal sides of one could become muddled and blurry, causing a feeling of non-humanity to develop and, ultimately, permanently stick. But I believe that, without a professional psychologist's perspective, this is honestly hard to determine and, even if it were true, is probably only relevant for a very certain sub-set of therians.