There is no malicious file in it

That is what someone uploading malicious files would say ;-) (sorry, could not resist)

No one will tell you, what flagged your game for closer inspection/quarantine. This is deliberate, so as to not educate the real malicious people how to avoid getting flagged.

It basically could be any reason, maybe a false positive on a heuristic scanner. If you read lots of comments on games, there are the occasional games that get flagged by anti malware on user systems. There can be assumed to be false positives on host side as well.

But to be safe, you actually should scan your own system for malware. There exists the possibility that your system is infected by some classical malware that infects executeables.