What brought you here 여긴 어쩐 일이야?

Expressions used to indicate an unexpected encounter of someone.

You don't want to say "why you are here? or what are you doing here?"

 

What matter/affair brought you here? What's going on?
What matter on earth brougt you here? (really surprised to see)
On what occasion have you come? (Usually used in an office for a visitor)