I noticed this among older Chinese people or also in the countryside.
One Chinese person told me that at school children had to keep their hand behind their back as a sign of obedience to the teacher.
Also thinking about it, keeping hands behind your back is part of a traditionnal 19th, 20th education. This education might come from foreign countries? After all foreigners were in China for some time. And Chinese education copied western education?
But there is more to it....
Question: what do Hitler, Mao Zetong and Kim Jong Un have in common?
Hitler holding his hands behind his back.
Today Chinese official often hold their hands behind their back as a show.
Thus they can look more like Chairman Mao. I guess they think it looks great... people are strange sometimes...
And of course Kim Jong Un.
He too copies Chairman Mao.