The translation note's explanation of straddling is a bit vague. Straddling isn't just something your enemy decides to do—it's something they work to achieve, by firing repeated salvos and correcting for errors in judging the target's range, course, and speed. When you've been straddled, it means that your enemy has corrected all their errors in your range, course, and speed and is now firing salvos whose shells are landing on all sides of you. That means they've narrowed down their firing solution to the point where they're likely to put their next salvo right on top of you...unless you maneuver now to spoil their solution.