As used here, the question "Why?" has a bit of a foundation in Identity View. There is an implied question of "Why am I suffering this kamma?". A key assumption of identity view is that there is an essential self that exists in our body. It would be this "self" that generates kamma through contact, feeling, craving, rebirth and suffering. It would be this "self" that would travel between this life and that life intact like a passenger transferring to a new bus.
And yet that very body has DNA inherited from our ancestors, who in their lives adapted to conditions at that time. There have been studies that some adaptations for a resistance to a certain disease such as cholera may make one susceptible to cystic fybrosis. In other words, our ancestral selves sometimes survive at our expense. In this case our current "selves" actually span generations of separate lives. The illusion of a well-defined self starts to break down and not work so well.
And just as DNA passes from one generation to another, so do behaviors. There are good and bad patterns of behavior passed down through generations. Parent hits child and child grows up to be an abusive parent. If one murders, one believes in murdering and that echoes forward into shorter lives for all (your example MN135). Here, too, one needs to consider where exactly the "self" boundary is. In this way, asking "why?" spins off into more and more tangled considerations, the so-called "thicket of views".
Instead of chasing the "why?", it is more productive and effective to simply observe that:
This is not mine, I am not this, this is not my self. --MN62
For example, instead of "why?", we might choose to say:
"This body suffers from cystic fybrosis, but this cystic fybrosis is not mine..."
"My father hit me and yet this hitting is not mine,..."