Some species of Spiders naturally stayed together for protection, collectively spinning webs in colonies of thousands. This was a natural delf-defense mechanism of their colony, both for protection in numbers as well as to alert their fellows 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.