Actually, young people *do* sometimes drop dead. A friend's 18yo son collapsed and died during a school sports day race, due to an undetected heart abnormality - she and her husband were watching.
In this case, the erratic behaviour would tend to indicate drug use, and the nose bleed could relate to a head injury when he fell on the pavement, or to overenthusiastic restraint.
Whatever the cause, it's a sad waste of a life.