I have a friend who has black out spells for no apparent reason. Why may this be?

my friend has had a traumatizing childhood. he has only blacked twice, but he sometimes randomly feels as tho he is going to black out. of those two times, this is what happened: once was when someone broke into their house and he saw a shadow, panicked and then blacked out and woke up with a cut on his foot. there was another time whenever he was in an alley with one of his friends and a black guy came up out of nowhere and started hitting his friend with a baseball bat.he woke up in his friends bathtub with his older brother splashing water onto his face. he hasn't seen any type of pattern when he blacks out or gets the feeling hes going to and it doesnt happen to him very often.