A Chain Swarm is when a zombie sights a potential target, and lets out a moan. This is then picked up by another zombie (typically close by, but range may vary due to environmental conditions ex. Wind factor). This zombie then lets out its own moan, which is in turn picked up by another zombie. Chain Swarms typically draw all zombies able to hear and answer the call within a given area. This can accumulate to bring dozens, hundreds, thousands, and even millions of zombies slouching towards a single point. If there is enough flat and open country for them to congregate in, they can coalesce in "mega-swarms": much like buffalo herds, they mostly occurred in the open plains of the American Mid-West and mainland Asia. Chain Swarms can travel from many miles away, as long as there's another zombie to continue the message. Some mega-swarms could grow so large as to be clearly visible from the International Space Station.
During the Battle of Yonkers, a Chain Swarm brought all of infected New York City upon the forces set up at Yonkers, ultimately crushing the ill-equipped and ill-prepared forces.
The Battle of Hope was a planned Chain Swarm, intending to bring in 'Zack' from far inland.
An early study theorized that this was a deliberate act, that a scout used its moan to signal the others to attack. However, we now know that it happens purely by accident. The ghoul that moans at the detection of prey does so as an instinctive reaction, not as an alert.