What is the frequency of a Cardiac Defibrillator going off?

My dad had a defibrillator put in 5 years ago, never had the defibrillator go off. Two months ago he had to have the battery changed & they implanted a new device. Four weeks after the implantation it went off, he was sleeping and that's what woke him up. Last night which was 4 1/2 weeks later we were out to dinner and it went off again, very scary experience for all. My concern is why didn't it go off at all over the last 62 months and now since the new one was implated it has gone off twice? He doesn't want to give up driving but I know he's worried because you don't know when it will happen again. I would love to have some advice/experience from a qualified user. Thanks in advance!!