He can explain!
 
One day while treasure hunting in the sea, Garble happened upon a sunken ship. It was loaded with valuables but none compared to the golden harp he found. He managed to haul it home without being seen and he cleaned it. He pondered melting it to create something more befitting a rugged dragon like himself, but the somehow perfectly preserved strings piqued his curiosity.  
After he plucked one, there was no going back. Life as he knew it was over and with him he would carry a dark secret he couldn’t even tell his closest relatives: an irresistible urge to play this dainty instrument whenever one was near. The worst part? He’s good at it.
 
This proves problematic in Equestria, which has a high harp-to-pony ratio for some cursed reason…