While I am aware that I have a more scientific approach to life, I decided to add my two cents. Note: while I may say some things that contradict other's points of views this is simply my point of view and I am not trying to tell anyone what to believe or think.
Psychological therians:
Therianthropy in general: I think that psychological therianthropy could be caused by the brain simply not being wired correctly. Now, as I'm saying this I would like to point out that what I mean when I say not "wired correctly" is that it is not wired how it should be/typically is, I am not saying there is anything "wrong" with therianthropy or anything of the sorts. I think that it could also partially come from feeling othered my humanity and so you subconsciously turn to animals, and animals dont judge. I think that with a combination of the brain not being wired correctly and possibly already feeling othered from the people around you could be what creates therianthropy in a person.
Theriotypes: I think that, if I'm correct about the first part, since you already feel sort of othered by humans and have subconsciously turned to animals and start maybe noticing more animalistic traits from your brain not being wired correctly. Then, since there are So. Many. Species on the planet most are able to find 1 or more animal that fits their description.
Spiritual therians:
I mostly think the same things with spiritual therians that I do with psychological therians but with one extra bit. I think that any opinion or religion or anything is product of how your brain is, and impressions on you. With therianthropy being no exception.
Again, I am not saying you should not believe what you want, ornwhat you should think, I am just trying to add what I think to the conversation.
Have a lovely day