It is not generally known why bronchitis in Yorkies occurs. To properly diagnose your Yorkie they will need to have chest x-rays and often a transtracheal washing.
Treatment can be a combination of medications including bronchodialators or corticosteroids as well as lifestyle changes. For instance, if your Yorkie is overweight, they will need to lose the weight. You should also use a chest harness for walks instead of a neck collar.
It is important to note that Yorkies suffering from chronic bronchitis should be kept away from tobacco smoke. If this condition is left untreated, it can degenerate into bronchiectasis and then emphysema, potentially lethal conditions.
As always, it is critical that if you think your Yorkie is suffering from a medical condition to bring them into see a Veterinarian.