Harry answered, "Because of magic instability?"
This is a phenomenon widely seen among young wizards.
The most famous example is Gryffindor's Seamus Finnigan, a boy who excelled at turning every kind of charm into a blasting curse. The magic within him lacked stability, so during spellcasting, it would yield to Seamus's most instinctive emotions. The slightest fluctuation would cause it to go off with a loud bang.
Chapter 389