[Anna Banana]

Its very common in yorkies because of their delicate little throats. Try doing a search, I know there is a lot of threads about it here.

In a nutshell, I believe it can develop from either inheriting it genetically, or from damage occuring from being walked on collars, etc. It is pretty straightforward, the trachea collapses, making it difficult to breath and causing the hacking that you noticed.

Talk to your vet about the possibilities of surgery, I think it depends on the severity of the case. You might also consider a second opinion on this option, as I believe vet opinion can vary on this issue.

You should be able to find a lot of information, and many people have experience with this here on yorkietalk. They will offer excellent advice.