Why do i cough so hard when its sinus drainage?

Cough clears mucous.: Your sinuses drain into the back of your throat. When you have a lot of mucous draining, your normal cough reflex is there to help keep the mucous out of your lower airway passages. This is why I usually don't recommend cough suppressants, unless it's during the night to help with sleep.

If the: drainage is running down the back of your throat it is causing irritation which then triggers coughing.

Irritation: The drainage down the back of your throat causes irritation that your body wants to get rid of leading to your cough reflex.