This issue has been discussed many times , and vets dont seem to have an answer for the problem. They have come up with many theories and many cures but nothing changes, I have 4 Shih Tzu kids and two of them are still sleeping in cages but they dont seem to have the problem yet, the older kids did it for a while and then stopped when they got to about 4 years ,for no exact reason. Although I did run ragged cleaning after them for about a year LOL. :D
And the problem is not restricted to any breed.THEY ALL DO IT.
Best of luck trying to solve the problem , If you do please let us all know, as Im sure I would like to stop my babies, who will not want to miss out on that horrible part of their development.