We hear of this often, and I have a "theory"
FWIW...
As all animals mature, there are several changes in their attitudes and appearance. The 4.5 years for Yorkies is generally toward their near mid-life.
In the wild, which is where they all came from originally, responsibilities of animals to provide for their Pack is placed on them. I believe that most changes in their appearances is related to better blending in to their environment, sort of Mother Natures way of providing camouflage.
The more stealthy, the probability of more productive hunts, and subsequently the Pack is better fed.