Last part we left off on was Part 1 "Why?"
I can continue with the next question:
I personally believe that a type of death comes to different types of virus's out there. So in the case of a human to zombie conversion a person will become motionless and the death of the human. In that time the body would prepare for the zombie phase of its existence and close the throat then. It also applies to cases where the human to zombie conversion is through a rapid change instead of an actual death. It all begins where the body prepares to become a zombie, a little after the brain kills of some of the functions it doesn't need will it be time to change the throat.
The tubes we breathe through are just like all the other muscles we have, they can contract. At the point of the conversion where the throat closes the muscles simply do what they can do - contract. There are a lot of theories about this one of them being the throat decays so much that it actually interupts the breathing process and creates a different kind of restriction.
There are more questions and this is open to discussion!