Deathwing kinda went through a bunch of phases of his body's deterorating and needing more and more plates and armor to keep himself together from ether his own battles, the Dragon Soul or his own immense Old God boosted power being too much for his body to handle. you literally see him being refitted some more in the Cata opening.
I presume at some point his lower jaw literally fell off leading to the giant prosthetic jaw he had in Cata.
Given what we saw in Dragon Soul at the end when his body finally gave in and came apart, he had a pretty good reason to continually refit himself.