RIR are one of the most popular breeds that hatcheries sell. Since hatcheries breed to meet demand for chicks, they pay less attention to things like temperament or how closely the birds meet the standard of perfection as their focus is mostly on breeding birds that produce large numbers of eggs (both so that they can hatch larger numbers of chicks and because a majority of the people ordering chicks are looking for laying hens). So, it stands to reason that there will be a seemingly larger number of aggressive birds in an extremely popular breed.
But you also have to consider what "aggressive" means in relation to chickens and chicken keeping. If you just got some pullets to lay eggs for your backyard flock you're most likely fine. The overwhelming majority of hens, even those from breeds with a reputation for aggression, are going to be docile towards humans. Human aggression in hens is not unheard of, but it is not that common and usually doesn't escalate to the point of injuries. "Aggressive" can also refer to how the chickens behave towards other chickens. The most aggressive bird I have is sweet to us and submits to us by squatting (like she would for a rooster) pretty often, but yanks the feathers out of the other hens if they don't get out of her way fast enough. I consider her to be "aggressive" to an undesireable degree, but at the same time I have absolutely no fear for my two year old's health and safety when he walks right up to her and tries to pick her up.