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.
Kind Regards, Salvo
When you are dead, you won't even know that you are dead... It's a pain only felt by others.
- Same thing when you are stupid.