A matter of reality/potentiality I presume. Using the perfect form 'have happened" implies that the event you're refering to has actually occured, whereas using a bare infinitve "happen" doesn't say much about whether the event in question has occured or not. It even conveys the impression that it has not occured, or might have occured elsewhere but not in this context.
Hope you'll get other answers and more details on this.
Student or Learner