Behavioural changes are very common in dogs and cats.
They result from contexts, situations and experiences that may cause them fear and stress.
Minor changes, many of which we cannot control, are enough to trigger all the mechanisms of stress and fear.
These mechanisms become more evident according to some genetic traits as well as past experiences that may have caused any kind of threat.
In the first contact with the threat, the animal shows an attitude of hyper surveillance, trying to solve the problem immediately (running away, barking, etc).