Before the story actually starts, Zootopia is in a "balanced inequity" where predators and prey live together and everything is mostly fine for now. But everyone has these unresolved prejudices just below the surface, which are ripe for being exploited.
It's been a while since I saw it ... I don't think the First Story Driver (the Action that throws everything out of balance) is actually shown in the film. I think it must be when Bellweather begins infecting predators with the evil toxic flower stuff.