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

When my baby was younger (under 6 months) he does this when he's tired and wants to sleep. It's one of his cues if we missed it, it'd be the start of a miserable night for both of us because he'll be too tired and too fussy to sleep properly. But nowadays, he arches his back and throws his head back when he was us to put him down too on the ground for him to "stand". I guess after a while, you'll figure out his own little special sign languages