Stress
Do you enjoy spending nearly every waking second trying to scrounge together enough cash for meals and rent? No? Well, then, there may be some stress. Also, unless you've got a hide that's tougher than a rhino's, auditioning is one of the most stressful things in the world. Even if you're great, you'll probably be rejected quite a bit.
Don't expect those nerves to go away with time either. They may, but there are many fabulously successful singers and musicians who admit they still shake like a bowlful of jelly before hitting the stage. Barbra Streisand is one famous example of a mega-diva who suffers from routine stage fright. Her voice may be "like butta'," but that doesn't make her exempt from getting "butta'flies."