I don't know why Kotori is always the subject of being yandered
I was thinking the exact same thing a few pages into this.
People always like to turn characters into their exact opposites. Kotori is the innocent, cute and understanding one. She's the character who goes along with everything and smiles gently at the group from the sidelines. It's not too hard to imagine that it might just be a complete farce and is actually smiling with sadistic glee on the inside.
Since she's calm all the time and is never shown to be angry, she seems like the character that no one wants to actually piss off because her angry side would be terrifying.
She's also pretty sadistic (makes short and revealing outfits for Umi to wear for her own pleasure) and manipulative (Umi-chan ONEGAI!). All these characteristics make her the perfect candidate to be a yandere.
She's imo the most blank slate character of the group with no really defining characteristics besides being the "nice" one so it's easy to make her the yandere since it's not necessarily against her character (and makes her a bit more interesting.)