My guess is that it would be psychological. In the few posts I've seen on "how did you become a zoophile", the running theme seems to be that when you are younger, in your early-mid teens, people are either exposed to it in some way or find it online. If this happens during a time period when hormones are high, your brain may associate the act of zoophilia with a sense of arousal, and further exposure cements that connection, until it becomes a part of your psyche.
I'm not a psychologist though. However, this is the prevailing theory presently on how people develop sexual 'kinks' and 'fetishes' There is an article published by the University of Richmond, which, while not a proper scientific report, at least gives a rough overview of how they form. You can find it by googling 'fetish formation scientific literature', it should be neat the top. Hope that answers peoples questions!