Smudge is a blues girl.

Smudge being the older cat of the household. Every guitar lesson she curls up in my teacher's case and meows along with the music, completely out of tune of course but ten out of ten for effort and cuteness.
Riff, however, freaks right out whenever I pick up my guitar. Hides under the bed till I've stopped playing. She's just turned one.
Any ideas as to why she freaks out?