This is a defensive posture they sometimes take when approached. I'm unsure if the intent is to camouflage better, or to better present the poison glands on their shoulders to a predator. Gulf Coast toads aren't nearly as toxic as cane toads (which can kill predators, or any animal that happens to lick one), but they can still secrete an irritant poison.
Regardless of the intent, the posture is funny. They can flatten themselves remarkably well, as frogs and toads lack full ribcages. It makes me think of a cartoon character who's been squashed by something, but only mildly annoyed.
I came across this fellow by surprise in our backyard. Since he was clearly not happy with me being near him, I left after taking a quick photo.
