This is an insightful question although it is also the case that karma will not necessarily act immediately. The transgressions of a lifetime may take a while to return through the wheel of karma and may skip a life in between and visit someone subsequently, particularly when occasions arise in a particular lifetime that are a match in some way to something that has happened before. That will create a clear opening for karma to weigh in and bring forward the energy that has been stored on deposit within the akashic records representing the suffering of a prior traumatic event, both that of the instigator as well as all victims.
By bringing up the subject, one is, in a sense, dialing in the address of the prior backlog of unhealed mischief, and that will come flooding in and add significantly to the burdens of the moment, and it may become an intolerable one, and the person might go under or may well have a very serious consequence that is life-disrupting. This is how karma works, it fits the occasion because everything is orchestrated by energy and the nature of the energy in question. It has a very precise way of assigning origin and meaning, both, so the energy within the karmic record contains information about all the specifics, and that is how it can find its way to the individuals involved, with precision, and even serve to bring back the players together once again to replay the drama as an opportunity to make amends, and often this results in a worsening, depending on the readiness to do better on behalf of those involved. This is a serious matter with serious consequences always, and people vary greatly in their ability to cope with karma and its consequences.