Night doesn't end "randomly", there are several factors deciding when the night ends:
- you clicked the "end" button next to the timer;
- you had low battery during the night (low stamina);
- you got caught by Yuna or Kan;
- timer has run out.
In your case, one of those four happened which took you by surprise, I think you either misclicked the end button or your stamina/battery was low.
Cheers