Theres actually a relatively simple explanation when you look at the neurobiology. First of all toxo is a parasite that can infect and damage the brain so it makes sense that it could affect behavior. Secondly, the response of fear to cat urine or attraction to female mice is mediated by the same pathways in the brain (the limbic system, which governs emotion, and is closely tied to olfactory input, specially in mice). Making the mice feel attracted to cat urine is actually not that farfetched when you stop to think about it and notice that its just a matter of "flipping a bit" somewhere in the wiring. And since this adaptation leads to a large fitness increase it makes sense that it would be strongly selected for that that the modern version of the parasite would have very specialized mechanisms to take advantage of this quirk in the rat brain.