Umaga's destruction came imo when he started jobbing like there was no tomorrow to Jeff Hardy. In his defeats to Cena, Lashley and Triple H it was made to be like a big deal and he was, in a way, kinda protected from it.
But then, he started losing in curtain jerkers with Jeff Hardy, getting pinned by just taking Whispers in the Wind and it all went downhill from there. His injuries kept him from being important on Smackdown. I sense he could have saw his career revitalized on the blue brand