Veterinarian manuals cite several possible sources of anxiety in small dogs. The main points include fear, separation, and aging.
Fear-based anxiety can stem from natural stress triggers in dogs or triggers that stem from some sort of trauma. Things like loud fireworks and other noises, strangers that look a certain way, new animals, new environments, or rides in the car can all be fear-based triggers that can cause anxiety symptoms.
Separation-based anxiety affects around 14% of all dogs, regardless of size. Dogs that deal with this type of anxiety struggle to get comfortable with being left alone or not with their stable family.
Aging can also cause anxiety in older dogs. This is usually due to cognitive degeneration, as well as neurological issues with memory, perception, and awareness.