What Causes A Dog To Hack?

I'm not a vet. But Ive had some different experiences.  1 reason..some dogs do this weird reverse sneezing thing.. Its got to do with their trachea especially in small dogs.. Its nothing to worry about but looks scary.  Another possibility is to check the dog for heartworms that makes them hack or possibly a cold.. Or something stuck in the throat.. Once had a dog get a piece of rawhide chew stuck.
Good luck..

Reverse sneezing is not a problem with the trachea and can affect dogs of all sizes.  Reverse sneezing is related to irritation of the nasal passage.  
A hacking cough is often secondary to kennel cough or another type of upper respiratory infection.  Cough can be secondary to heartworm disease--this is more likely if you are in an endemic area.
The best thing to do is have your dog examine by a veterinarian.

Heartworms will cause a dog to hack because their lungs are collapsing. You need to get her checked.