It just came to me: Obi-Wan said that Vader was "more machine than man. Twisted and ee-vil." So, it was the "work" that Vader had done. First, to get remove those red eyes, then a new hand here, a stronger shoulder there, a little off the tummy. He heard about HGH (he's getting heavier) and staroids, but failed to consider the side effects. So, to answer this thread's questions, two words: 'roid rage. One last word to prove it: NNNNNNNNNOOOOOOOOOOOOOOOOOOOOOOOOO!!!!!!!!!!!!!!!! !!
Or, on a serious note, perhaps with the Rebellion's victory at Yavin, the Empire increased its call for troops and supporters, thereby bringing in lesser quality and experienced soldiers that could get promoted (even due to nepotism or bribery) and infuriate the Dark Lord even more.