Cats are social creatures; contrary to popular belief they are
not animals that prefer to live alone. While they can adjust to
a solitary life, they also exist quite happily in a large
family, whether of people, cats or both. Cats are, however,
creatures of strict routine and habit. They don't deal well with
change. A number of factors can affect a cat’s social position,
including the introduction of a new person or cat to a home
which can cause a great deal of stress. Cats have a very
structured, formalized "social tier" and a great deal of their
emotional well-being comes from knowing where their place is on
the social ladder and knowing that their place is secure. As
long as social position on the ladder is in flux or open to
challenge, your cats will be in a state of high level stress.
(Think of it as if the company you work for has told you they're
going to fire an employee – just not which employee. The stress
from the threat of job loss stays with you at ALL times, always
prevalent, not just at work).