This is an old story with CZ rifles. - If somebody adjusts the trigger too light, it will either fail to cock ( firing pin not held back by the sear in the first place ) or it will act as the gentleman described, where you cock it with the safety on, and it fires upon releasing the safety.
A lot of shooters do not want to use the set trigger function, and try to adjust the regular, unset trigger down. This is where they run into this problem.
I know because it happened to me with a 527 Varmint, and showed up when I tried to test the trigger by dry-firing... I un-adjusted the trigger again, and all was well.
With the gun empty but cocked with the safety off, give the butt a few good bumps on something to see if you can jar the sear engagement loose. - You don't want it to be so sensitive that falling over or being jarred will make it fire.