I think it came from the fact that you place someone on a pedestal and call them incredible..........and eventually it will affect their ego, and teh effect will be an egotistical person who is full of themselves. No doubt, Brando was a very very accomplished actor, and his earlier films were great.
however, ego wise he was almost unparalleled as well.
Superman - demanded, and got, the most incredible pay package for the shortest cameo at the time. I don't want to think what "modern day" conversions of figures would be, but astronomical would certainly be around the right mark
Appocolypse now - it was because he was ashamed of his size and appearance, and the effect it would have on him from public perception of him that made him be restricted to the end of the film, and always in the shadows too. He didn't want the adoring public to think of him as a bloated fat ugly man, he wanted to be remembered as a youthful good looking guy. It works too, as i cannot really think what he looks like! Unless i remember him in Godfather, my memory is of him in younger years