This is not a definitive essay on the subject. It is not an article. It is just a post.

First, a few assumptions on which this is based:
reincarnation happens
the base of being, the soul, if you will, does not differ from species to species. Differences are generally from being to being, with some developing similarly when exposed to similar situations. Alternatively, remanufactured souls can exhibit very similar structure to each other, and are those that are some of the most likely to deviate from the 'basic soul'. The next most likely is the fully manufactured soul, which may deviate most extremely for (hopefully) obvious reasons. In any case, the base of this assumption is that all beings are based upon the same thing.
I may have forgotten a few assumptions, so please keep in mind this is built upon a paradigm that is likely deviant from both the norm and your own.

That being said, it rules out the 'wolf in a human's body' reasoning; seeing as under this paradigm you are simply 'a being that was in a wolf's body now in a human body.' So, the proposed reason (under this paradigm) for 'otherkinism' is that the being experienced a lifetime in whatever form (to use the stereotypical; a wolf). Now, that alone is usually not enough to incite an otherkin reaction in the lower consciousness of a person (that which develops in individual incarnations, and is 'reabsorbed' by the total consciousness of the being upon death or whatever transcendence), if it is, it is usually due to the most recent lifetime being of that creature. The more lasting otherkinism is caused by deep trauma that not only affected the lower consciousness, but the total consciousness -before- reabsorbtion. This often depends on the integrity of the lower consciousness, and can vary from violent death to a partial remerging during an incarnation of the lower and total consciousness and then suffering a severe emotional upset. The psychological traits and sometimes soul shellform of the lower consciousness is then instilled in the total consciousness, later manifesting itself in another incarnation. This typically only happens when the wound is fresh and the total consciousness has not normalized before manifesting the lower consciousness, or when the lower consciousness establishes a solid, powerful link with the total consciousness in the course of an incarnation.