It's not only that. Moeko was tired the first day and wanted to skip beach and start studying, but other girls, especially Yamada, wanted to go. Next day, her food/meal plan was upset by Kyotaro wanting to show off, and he even got praise for that, while her own curry went ignored like it was normal for her to be the group's chef. Still, she wanted to play nice and complimented Kyo, but then Yamada got really weird about it "I must eat all of my boyfriend's soup, give it up!!!", which is such annoying couple behavior. Remember that Moe was also burdened all the time by being their secret-keeper before the rest of the group, which probably also bothers her, especially when Yamada starts behaving like that.
And then, when they finally started studying like they were meant to, Yamada, the indirect reason for the need to go shopping, instead stays clearly to keep an eye on her, and starts bothering them during study, and this bothering takes form of "you're working so hard on this trip, you are so great as a girl, I wish I can help you more". For fucks sake, you want to help her, stop fucking getting in her way all the time, including right now, and actually help out! That really makes it like Yamada doesn't mean what she says, even though she does mean it, because it doesn't fit with how Yamada actually behaves.
To sum it up, it wasn't just staying behind that pissed of Moe, it's staying behind while bothering her with "I wish I could help you" when doing the opposite of helping her right then and from the beginning of the trip.