Why does shouting cause hoarseness in your throat?

Trauma: Shouting causes local trauma to your vocal cords when they slam together trying to make a loud sound. This causes swelling and inflammation of the vocal cords which results in a hoarse voice.

Traumatic vibration: Your vocal cords vibrate hundred of times per second. Normally this repeated vibration causes no ill effect on the voice. If you were to yell, you now overload the system and the vocal cords will aggressively hit each other hundred of times per second. Even with proper training repeated yelling could be detrimental to your voice.