Coffee breath is caused by the chemicals in coffee slowing saliva production. The lack of saliva creates a perfectly undisturbed environment for the bacteria which already exist in the mouth to multiply as rapidly as possible.
Plaque contains 300 types of bacteria, some more malicious than others.
Bacteria grow exponentially...so the more bacteria you start with, the more you end up with, and the smellier you become.
Note: Eva is not a dentist. She is an educated consumer using public information and basic logic from WebMD and Scientific American