Well I mean, I could see how something like that would be seen as too narrow in focus. Talking about buying a specific practice from a specific vet is quite different from having a clear, focused interest in an area of veterinary medicine. I mean, it seems a lot more inflexible to say that, I guess. They could certainly see it as you not wanting to be open to anything new - sort of like the way that "academic incest" is frowned upon in academia.
Maybe you could frame it a different way, that working for X doctor has really inspired an interest in the mixed animal field because of this reason or that reason which connects to OTHER experiences/personality traits and you're interested in owning a practice one day because you are interested in running a business and etc etc.