As to why Peng was ill treated during the CR, I believe, as SOB also suggested, no one stood up for Peng, not one old marshal dd. Peng had alienated so many old marshals and his generals.
Marshal Yep and Zhou Enlai stood up and spoke for Hsiao Hua and Hsiao escaped from the public humiliation in the early going. Mao gave up on Hsiao on the alleged rape among other things later on. No one did it for Peng.
v5- The fact remains that Mao was not among the equals of Peng. If Peng were to lead the Red Army in Long March or Long Escape and the civil war, Mainland today would be what is like today’s Taiwan. The problem with Peng was his hard temperament. Peng didn’t respect Mao, Zhu De and other old marshals. Peng’s character would not survive (not necessarily death) in any country and under any political system. What would happen to Peng if he treated Chiang Kai-shek as his equal and disrespected Chiang? What would happen to Peng if he treated Stalin and Zukav as his equal? What would happen to Peng if he treated Roosevelt and General Eisenhower (European Theatre) and General MacArthur (Asian Theatre) as his equals? Truman who had no military background dismissed MacArthur for insubordination which was a very unpopular move at that time. In a way, Peng was his own worst enemy.
Do –U - Read : I am very reluctantly to believe that Peng was anti Mao. But the facts pointed to the conclusion that Peng was to some degree. As to what degree, who knows. It is obvious that Peng’s letter was premeditated and I doubted that Peng drafted the letter himself. Who drafted the letter? Peng’s secretary or one of the “hsiu tsai”. Tien Cha-ying was involved very deeply in the matter. Peng showed the letter to Huang, Zhang, Zhou and discussed the letter with them before delivering it to Mao. Peng’s letter coincided with a series of Russian anti Mao aactivities. I am saying that Peng colluded with the Russians. But the suspicion was rightfully there.
The above were my personal view and understanding and nothing elase.