What causes this action?

[AussieChap]

I have two cats. When I take one to the vet and return home they hiss at each other and growl. This has happened several other times. They are brother and sister, 15-years-old. Have always been indoor cats. What's causing this. It goes away in 3-4 days.

[Mollycat]

They smell of the vets. I know it sounds weird but scent is very important to cats and they are very territorial. They reinforce the family bond when they share their scent, by rubbing and mutual grooming, if they do that, and suddenly they smell different. The breeder I got my retired stud boy from would have loved to keep him as a pet but her queens would not tolerate him because after neutering he smelled different. Even if yours don't physically touch each other they still know each other's scent and accept it, and any change makes them wary. My other cat Molly, for a couple of years after I adopted her, would wash herself thoroughly after she had accepted any petting from me to clean my scent off herself. As long as it's just a bit of hissing and growling and settles in a few days it's just something to watch so that it doesn't progress to them falling out with each other.