Most types of Rats naturally stayed together for protection, huddling in small groups during their sleeping hours to defend their nests. This was a natural delf-defense product of their colony, both for protection in numbers as well as to alert their fellow Beasts of any danger.
Certain types of swarms, however, were not benign assemblies of little creatures. They formed as a result of a sinister or unwholesome influence, and would not leave the swarm even when in danger. Even powerful druids could not dissuade or charm these swarms, who's aggressiveness was borderline unnatural.