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Former Guizhou party secretary Chen Min'er, a protege of Xi, was announced as his replacement earlier on July 15.
"His removal demonstrates Xi's solid grip on Chinese politics, particularly personnel arrangements, in the lead up to the 19th Party Congress in autumn," Willy Lam, adjunct professor at the Chinese University of Hong Kong's Center for China Studies, told CNN.
The party congress is expected to re-elect Xi for a second term as leader while announcing the new makeup of China's top decision making body -- the Politburo Standing Committee.
"According to party tradition, (Sun) would have been elevated to the Standing Committee," he said.
London-based consultant Trey McArver said in a report Sun had long been discussed as a possible frontrunner to replace Xi as general secretary and Chinese president in 2022.
"Sun's ouster is instead the latest sign that Xi will not be bound by rules or norms that predate his leadership ... anything could happen at the upcoming congress," he said.
Playing politics
"He is a powerful figure, he is the most powerful leader since (late paramount leader) Deng Xiaoping," Cheng Li, director at the Brookings Institution's John L. Thornton China Center, told CNN in April.
Sun was not considered part of Xi's power base and was seen as more of a technocrat, not affiliated with any major party faction, Lam said.
"I think one of the most important reasons for his downfall apart from these allegations of corruption was in the past five years, since Xi became leader, Sun has failed to demonstrate full allegiance to him," he said.
In comparison, his replacement Chen has long been a close ally of the Chinese president.
"Chen -- as head of the Zhejiang Propaganda Department -- helped Xi to craft his image when (he) ran Zhejiang from 2002 to 2007," McArver said.
Analysts say Chen is now expected to rise into the 25-man Politburo, one step below the Standing Committee, helping Xi's allies to build a majority inside the group.
Poisoned chalice of Chongqing
Sun is the second Chongqing party secretary to be removed under investigation in the past decade.
He had been seen as a direct rival to Xi for the top spot.
Chongqing is one of China's largest cities, far from the political centers of Beijing and Shanghai and deep in the country's economic heartland.
For Chen, being given the opportunity to manage the provincial-level city of 36 million residents could seem both a blessing and a curse. But Lam said he had one big advantage over his two predecessors.
"His political identity and affiliation is very clear. He is a top protege of President Xi and so he of course has absorbed the lessons of his two predecessors ... (They failed) to align themselves with the powers that be," he said.