Bloggers going after (and in many cases, verbally attacking) a particular company or person who has done something egregious.
Imagine a swarm of Africanized honey bees (aka killer bees) swarming an unsuspecting victim who has agitated them by entering their territory, thus threatening their safety and way of life.
One well know incident of blog swarming was Vincent Ferrari v. AOL, which did everything they could to prevent him from canceling his account. As a result, Ferrari released his tape-recorded ordeal and many other bloggers and consumers swarmed AOL.