This is a tricky one: I wonder if it’s possible to have a more meaningful explanation for this force bond/dyad than just ‘it happens, but it’s unusual’. TROS barely has an explanation, but at least there Rey is palpatine’s ‘granddaughter’ so it’s the grandchildren of vader and palpatine being connected in a sense. Maybe, like so many things that happen because of the force - such as force ghosts - it simply can’t be explained and is there because the story needs it. I must say I’m really interested in the plot point of Rey accidentally finding out about the planned attack because of the force bond.
The best I can come up with would be that Kylo and Rey unconsciously feed upon the living Force, and their first encounter in episode VII inadvertently sparked a reaction which resulted in their Force bond.