I speculate that he may have been triggered by a humiliation that had nothing to do with his victims as individuals, but a humiliation that he associated with them in the abstract (i.e., as undergraduate students). I think he had selected his targets (either 1122 King or specific occupants of 1122 King) well before the trigger event, so the trigger event did not determine who he killed or why, but it did influence when he chose to act. (This is all just my hypothesis based on a possibly flawed understanding of BK's temperament and what motivated him).
The specific event I'll put forward as a potential trigger is the one described by WSU student Hayden Stinchfield in a NYPost article and interview:
A former student of accused Idaho killer Bryan Kohberger said the teaching assistant seemed “preoccupied” and started grading leniently following the murders of the four University of I…
nypost.com
Based on what I understand about BK--including the fact that he clearly identified as and wanted to be seen as intelligent--I believe the confrontational debate over BK's grading practices as described by Stinchfield would have caused BK acute humiliation. I believe BK has a rather authoritarian personality, meaning that he is inclined to show unquestioning obedience and respect to those in positions of power/authority while being strict or punitive towards subordinates. I think that when the (male) professor for whom he was a teaching assistant gave undergraduates the floor to openly critique BK's grading methods/practices, BK experienced a deep wound to his ego that he was not equipped to manage through his usual (limited) coping strategies. And, given his authoritarian mindset, he would not have blamed the professor but the undergraduates. I think he projected his rage at the undergraduate students at WSU onto the 1122 King residents (also undergraduate students) that he was already targeting for other (unknown) reasons. I think this event may have been the trigger that provoked BK to act when he did. (Unfortunately we do not have precise information about the timing of this event, but I would very much like to know when this occurred.)
In this hypothesis, it is BK's internal humiliation and rage, rather than external circumstances or the comings and goings of his victims, that really clinched the timing of the murders. And to be clear I think he would have attempted the murders regardless -- he had been fantasizing about them and planning them for a while.