Why did she go onto the balcony?

I think that was her way of saying "I'm sorry" and being humble. They were surprised for a moment that she would do that and then started up again.

I don't know why, but I took that gesture as lying her head down on the chopping block. That she would die for France. I'm probably wrong about that. Just a crazy notion I had. But, considering the way she died, maybe she just knew it was going to end that way. JMO.

"oooooh....someone needs a RAID enema to kill that bug up their ass!"

The balcony scene is based on a historical incident in which the mob demanded to see the queen. The exact details escape me, but I think she appeared first with her children (probably trying to present herself as a mother of France). They demanded that she appear without the children, so she came out again, this time guarded by the Marquis De Lafayette. Again, the mob demanded to see her alone. Finally, she exposed herself alone and unguarded on the balcony. Seconds earlier, the mob had been calling for her blood, but the sight of their queen, noble, unafraid, humbling herself before them was so overwhelming that they fell silent, and in a short-lived surge of loyalty, began to cheer for her. I think her bowing to the mob was a poetic invention on Coppola's part, but I'm not sure.