Something must have happened. We can speculate what nature is it based on the reaction by the party.
- Something pretty serious happened, seriously enough to cost him from a high position.
- Not serious enough to face jail or removed from party. He was simply demoted.
- Seems a bit severe for just extra martial affair.
- Too light treatment if he leaked intel.
- He looks young and healthy, questionable if his health is so bad from covid.
- If he did have bad health, then either he can continue working, or he should leave. Demoting don't make much sense.
- This seems very unexpected by the party. There was no suitable replacement and Wang Yi has to come back to fill the job from a higher position.
- Therefore it cannot be a power struggle. Otherwise someone would be promoted into Qin Gang's position, not have Wang Yi come back from a higher position.
I guess it could be a combination of extra martial affair and some other minor corruption issue that messed with his image, making him unsuitable to represent the country. His ability is still considered good, that is why his replacement is Wang Yi who is his superior, not someone below him. Whatever happened was not severe enough to get him booted from the party. Stuff like espionage and treason is out of the question.