Once u complete everyone in story mode, and u face off Bunta,
the game "resets" the opponent list, regardless of whetever u
actually beat Bunta or not (although the large amount of points
from defeating him does help in tuning a car).
the second time round onwards, u can manually choose to face off
Bunta without whacking everyone else 1st. Once this is done, u can
choose to face off Bunta in the rain. The game will not "reset" as
long as u have not defeated everyone else in LOTS yet