Basically, a big, big, man has fell down. And for reasons not yet clear.
He was de-facto a shadow minister for state security, and a man whom Xi would've never allowed even close to state power unless he was 1000% confident in him.
Many think that China has ministry of state security as its secret police organ, but in reality there are like 5 or 6 tucked around different state organs.
State council had its own secret police organ, people armed police also had a secret wing, but it was largely axed because provincial cadres abused it, PLA had 3rd department (now reformed into strategic support force,) CCDI is a de-facto secret police by itself with power to arrest even a provincial governor in a secret jail without having to report to anybody.
Public security also has a "national protection" section, but in reality that section is completely independent of the PSB command chain, and kept secret from police officers. They listen directly from Beijing. That man was heading it at the time of his arrest.
Sun was a very bright cadre, with a good track record on policy initiatives, an upstanding bureaucrat, if not to say too ambitious. People were saying he could be on the road to state council or the politburo in 2022.
Very hard to guess what made him fall. CCDI reports to one man only, so only Xi himself could've given blessing.
I don't think that a man like Sun was suicidal enough to stand against Xi. I also don't think that Xi would've done this during a "routine" purge of bureaucrats not sticking to the line, especially not at the time like this.
Three possibilities:
- Sun actually pissed off Xi, and this is his penance for that particular incident
- It is a "kill the chicken to scare a monkey," and an attack against somebody close to him
- It still can be a just routine anti-faction action to deny everybody not 100% on board with Xi access to future political promotion