It might not be "issues" per se, but just the amount of time they need for maintenance.
When they started opening with Park Open, they then closed the ride around 7:00 (after putting it
@Capacity at around 5:00) because they needed that amount of time from around 8:00 pm onwards for maintenance until the next morning's park open.
But I'm guessing if its pouring rain out from anytime after 7:00 pm it delays when they can start their nightly maintenance, which I would guess means it opens later the next day.