|Well, I think it's pretty hard to tell, who was actually falling first in this case, but the reason people tend to think it was Futaba could be the point of view. Because ...
Spoiler (mouse over to view)
Since the main focus lies with her, the reader gets to know right in the beginning many different details about Kou. These could be things, that lead step by step to the conclusion, that she started to develop feelings as the first one. For example, as she realizes the (from the other guys) slightly different way he behaves, and - more obvious - The even smaller details, like his scent, which may in comparison to each other be seen as a subtle way to express the characters interest in getting somewhat closer (not only in a physical way).
Although, it's just a guess. ^^"
"It's like love is a bonus in your life. Even without it, I'm just fine. I don't feel like pointlessly searching for it. You don't have to make yourself unhappy." (Ookami Shoujo to Kuro Ouji)
"Why does it rain?" - "To create drama." (Drops (Andou Yuki))