We had a dog who didn't like men in particular, and really disliked either black men (though not black women), or men in brown uniforms. My family most definitely did not teach the dog such dislikes.
In that area, the power company meter readers wore brown, as did UPS. UPS guys didn't tend to go into back yards, though, while meter readers tended to take shortcuts through adjacent properties. So, I suspect a meter reader may have kicked or sprayed our dog at some point.