It's mostly because of how she reacted to Keith when he brought it up, and that later on she was mad at Shei for spilling the beans (if she was telling the truth, then wouldn't the anger be directed moreso at Keith, who would have been straight up lying about them hooking up in front of everyone and accused her instead of lying?).
Mostly though I didn't think much of the drama in the house - Chantelle is aloof and defensive for sure, but understandably so if she was bullied as a child, but at the same time I don't feel like the rest of the house was super mean to her or anything, they seem pretty normal and courteous with her as with anyone else there. So it's also understandable they wouldn't or shouldn't feel like they have some big responsibility to treat her extra nice and be extra friendly with her either. The fact is, they all bonded with each other over the course of the competition while she was out very early - so I feel like a lot of it has to do with the circumstance - you have this group that have become close and then someone they barely knew came back, obviously they wouldn't be as close to that person than to the rest of the group, and it's not really the fault of that person or the group that they're not friends. I feel like that's all there is to the drama really, small potatoes really compared with other seasons.
But, Yu Tsai can just go screw himself though. I thought Chantelle handled it best she could and Yu Tsai was just super gross, which means the show is super gross in backing up that pathetic excuse of a reason. It doesn't matter if Chantelle has attitude problems or not, that's entirely irrelevant - Yu Tsai was out of line, period. And it's weird, because you'd think Tyra would have jumped on it, have the 'it's not ok to use slurs' PSA kind of thing, and tell us all a story of how she dealt with photographers/clients calling her names and stuff and how she rose above it all to be awesome. Such a missed opportunity, Tyra.
edit: Is either Shei or Adam korean or even part korean? Purely speculation, but they don't seem like they are.