Why do babies arch their backs and throw their head back when they're upset?

Agreed with what Stephanie said. Usually that means the baby is frustrated by something. Unless the baby does that during feeding. Arching back during feeding could be a sign of reflux or GERD. Fortunately, babies are quite easily distracted. Even if you can't identify what was causing distress, give him/her some attention and engage your baby with some other activity may distract him/her from the original source of frustration.