As the first rival said, they are looked upon as a group, so if she's shabby she's bringing the whole group down and so now the group have a reason to help her. That's just an excuse though, all the rivals seems very sweet and lovely.
We already know from the first chapter that they're pretty nice, as shown when they seem to pity her when MC looks very uncomfortable with the prince. They likely think that she's forced to be there and so want to help make her as comfortable as possible.
First rival is best helpful tsundere tho. Very lovely, that's how you do sweet tsunderes