Although humans, animals, etc., can learn to perform different behaviors, there are times when they stop performing those behaviors in the way they learned and start reverting back to their more instinctual behaviors - this is the basic premise of Instinctual Drift. The animal no longer performs the behaviors it has been taught, but goes back to behaviors that are in its nature. It begins to do what it is driven to do regardless of the resulting punishment.
For example, a dog with the nature to bark at visitors thinking they are intruders might have been taught to sit quietly when a guest enters through reward and punishment. Under stress, however, it may have instinctual drift, disregarding the learned behavior and barking at the guest.