Sure it is. Rats are very intelligent animals with complex social structures. When housed and cared for according to their needs they develop complex hierarchies among one another, which generally results in a peaceful living situation and happy, well-adjusted rats. Shove a bunch of unacquainted animals together in a cramped, unfamiliar environment and there's going to be problems, regardless of species. Nobody would put a bunch of strange, unsocialized dogs in a little room together unsupervised and expect them to get along swimmingly.
I always find it a little strange when people have a deep appreciation for one (also very commonly misunderstood) animal but call another disgusting because of its nature.