When does a reflected Junkrat mine explode?
Usually Junkrat can throw a mine that is triggered by the user.
If the mine is reflected by Genji when does it explode?
Is it user triggered and if so by which player?
It's still Junkrat that triggers it.
If he sees a deflect he can (and should) choose to not activate it, but usually in the heat of the battle you click the button before realizing what's happening.