As a therian I don't personally identify with the alterhuman community. I don't think therians need to be grouped in this way and I don't think anyone who identifies purley as alterhuman actually understands what therianthropy, otherkinnity or other of the "sub-group's" experiences actually are, or what makes a therian a therian.
However if you're asking what I think my therianthropy is caused by, and just worded it poorly...
I believe therianthropy is an example of
convergent evolution. I think our human biological traits and learned (aka environmentally introduced) behaviours come together to make us more similar to the animal of our theriotype(s) than other humans and that this is therianthropy.
We are biologically (genetically) human and we live human lives. These are the things that affect who we become. Because we live in the same environment as certain animals, we are inclined to come up with the same solutions as them to life's challenges. This can make us so similar to them, that living life in accordance with how they do might feel much more natural to us than forcing ourselves to do things as other humans do.
I believe this effect has been present throughout history, but it's been only recently that humanity has felt a need to classify and label every aspect of our identity, that we've given it a name: "therianthropy".
LP,
Dusty