Overly Intelligent people

Just got back home after witnessing a crazy man filming and shouting at people about how the people in the school that I go to is just laughing at him when he was just tryna meet his old mates which he is not cuz he's crazy and shit he's a crackhead but the smart type so I asked my friend about who that man was she said that he used to be overly smart and used to go to my school but he got crazy which got me wondering why some overly smart people ends up mentally unstable...