Perhaps it's part of a magical experiment. Fleur, being part - Veela, might have some unique magical abilities. By straddling Hermione's waist, she could be transferring some magic or energy to Hermione. This could lead to Hermione discovering new magical powers or spells, and they would then go on an adventure to explore these new abilities further.
Well, it could develop into a story of deep friendship. Maybe Fleur was trying to comfort Hermione during a difficult time. They might start sharing their secrets and fears, and through this physical closeness, a strong bond of sisterhood could form.
Another possibility is that it's a misinterpreted situation. Fleur might have tripped or been pushed accidentally and ended up in that position. Then, there could be a lot of embarrassment and awkwardness at first, but it could also lead to some humorous exchanges between them and ultimately strengthen their relationship in a more light - hearted way.