Why do people shout when they hear the Quran?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

Not all people shout when they hear the Quran. A person may shout when hearing the Quran because of the fear of Allaah, or because of sickness. Ibn Al-Jawzi reported that Zuraarah Ibn Al-Awfaa performed the Fajr prayer and when he finished he heard someone blowing the trumpet, he sighed and breathed his last [as he was very terrified]. 

Allaah Knows best.