"Hvad skete der med dig i går?"
Translation:What happened to you yesterday?
Not sure I understand what you mean by "just a phrase"?
I believe it would always be more sincerely asked than the English/American "How are you?" greeting, which I have discovered is used completely regardless of the fact that the other person has no interest what-so-ever in knowing about your current state (physical or mental).
This question would certainly demand some kind of answer pertaining to events occurring yesterday, whether the person asking is actually looking for insights or is using this sentence to insinuate that the other person might have done something wrong yesterday. By the latter I mean that it could certainly be used as a "what the f... was wrong with you yesterday?" where you might be looking for more of an apology than an explanation :-)