What is Happen by?
Happen by definition and meaning on Dictionary terms:
verb (used without object)
to take place; come to pass; occur: Something interesting is always happening in New York.
to come to pass by chance; occur without apparent reason or design: Don’t ask me what caused ita??it just happened, that’s all.
to have the fortune or lot (to do or be as specified); chance: I happened to see him on the street.
to befall, as to a person or thing: Something dreadful has happened to me.
to meet or discover by chance (usually followed by on or upon): to happen on a clue to a mystery.
to be, come, go, etc., casually or by chance: My friend happened along.
Slang. to be very exciting or interesting: That party was happening!