Based on the past two images it seems like Douki-kun developed a crush on her first, she rejected him and years later the situation seems to be reversed, but with Douki-kun being actually interested in someone else.
Which has happened to Maiko before, as seen in post #4500892.
There's also a slight implication from the flashbacks that her coming on strong did make him blush at least but the last two show her cute side more and it makes me wonder if she's the type to approach someone casually with ease but retreat once her flirtations are reciprocated.
I'm guessing that there was a time gap between the mixer/baseball cage scene and this confession. They're both wearing light summer clothing during their date, but here it's clearly autumn. Think Douki-kun had feelings for her but missed his window, which is pretty appropriate considering the theme of this entire series.
In Senpai/Maiko's defense, she liked him, and he liked her at one point, but she didn't want to cheat and stayed faithful to someone who turned out to dump her at some point. She wound up coming onto Douki-kun on the rebound, but it didn't work out possibly because he already had gotten over her, and he's definitely going for Douki-chan at this point. It's something that could have happened, and it's not like she did something wrong (not before coming back and being aggressive when she knows Douki-chan is in the picture, anyway), it's just a tragedy of bad timing for her.
