It might be the smell from the vet, but it might be something else.

Whenever Pichu or Milo shows signs of being sick, especially if they yowl, Juni would jump at them and start a fight. It might be something survival related, I've only heard of female kitties doing that. In the cat world, the kitties are the territorial ones and it might be that she is reacting against having someone sick around.

When I bring a boy from the vet, or when I need to give them a shot or a pill, I always make sure that we are out of Juni's reach. If not, all hell breaks loose.