happen作“发生、碰巧”解,常用于偶然或突发性事件,例如：
What happened to you?(一般不说：What did you happen?)
Maybe something unexpected happened.
I happened to see him on my way home.
= It happened that I saw him on my way home.
come about表示“发生、产生”,多指事情已经发生了,但还不知道为什么,常用于疑问句和否定句,例如：
When Mother woke up,she didn’t know what had come about.
I’ll never understand how it came about that you were late three times a week.
Do you know how the air accident came about?