Why do the Rainbooms go elemental when they play music?

[Soaring Symphony 372]

There is one enormous nagging question I have about Rainbow Rocks (and EqG to a lesser extent).  Well actually it's two questions that are sepperate but still interconected.  Why do the humane six turn into antro pony things at all?  And why does it only happen when they play music?  I can accept the Idea of the Humane five (not counting Twilight) having Equestian magic in the first place because they got zapped by the element of magic at the end of EqG.  The question is, Why would that magic have ties to pony ears and tails and stuff?  And why does it only "activate" (so to speak) when they play music?  any ideas?

Edited January 24, 2015 by The thoughtful brony

[Shenron00 366]

Well, when Twilight left, some of the Equestrian magic was left behind. When they play music, it unleashes the inner magic that they still have inside them. Why? Because music is supposed to reach into your heart, bring out the best in you, show your true feelings, or something along those lines. Why do they turn into pony people? Simple: the Equestrian magic.

That, and I really don't know what else they could turn into...

 

At least, that's how I interpret it.