Amid the 95-degree heat suffusing the hellish epicenter of New York City, a swarm of bees 30,000 strong descended on a Times Square hot dog cart on Tuesday, prompting the intersection to shut down. The surreal images, live-streamed from the scene by Reuters, might as well be figments of heat-induced delirium: Bees carpeted the awning so thickly that a masked, gloved man from the New York Police Department had to be called in to suck them up gingerly with a vacuum.

It’s not something you see every day, but it’s also not something no one’s ever seen before. Such immense swarms, bee experts explain, are usually a sign of change in a nearby hive.

“There’s two reasons why it happens. It happens from overheating or overcrowding, or a combination of the two,” Andrew Coté, owner of local company Andrew’s Honey and beekeeper, tells Inverse. Coté was on the scene for three hours with NYPD officials, helping to deal with the swarm. “Today was over 90 degrees, so it might have been both.” He notes that bees are usually docile when they swarm, though you wouldn’t have guessed it from the way Times Square visitors were reacting.

In recent years, a number of massive bee swarms have occurred in New York City that the NYPD’s “Bee Cop,” Officer Darren Mays, had to manage with his trusty vacuum. In an interview with the Wall Street Journal in early August, shortly after he tackled one swarm at a local pizza shop, Mays explained that the reason tens of thousands of bees stick together so closely is because they’re following their queen, who’s scouting locations for a new colony.

Bees normally swarm in the spring, says Coté, so Tuesday’s swarm was a bit unusual. “But if it’s a crowded hive and extra hot, those two together, that’s when it happens,” he says.

In other words, this is a perfectly natural phenomenon that only feels apocalyptic because humans are all up in the bees’ space. Speaking to the Wall Street Journal, former NYPD detective and beekeeper Dan Higgins said, “In nature, it happens in the middle of the woods. In New York City, it happens in Grand Central or Park Avenue where there are 500 or 600 people.”

There’s not much on a hot dog stand that might draw a bee colony, but Coté notes that a few buildings in the Times Square area that keep beehives on their roofs might be the source of the swarms. When the colonies in those hives are not properly maintained, he says, swarms can occur, even though they are usually preventable. “Bees don’t swarm without warning. If a beehive is regularly maintained, those signs are seen, and actions are taken to thwart the swarm,” he says. “It’s in their nature to swarm when it’s too crowded or too hot.”

Around all the people in Times Square, the bees are a nuisance, even though their motives for swarming are generally peaceful and they are nonthreatening. Bee vacuums are meant to safely move bees away from people into a temporary hive, which allows them to be transported to a new hive or released in the wild, where they have a better chance at finding a new one.

As explained in a 2006 article in Behavioral Ecology, several hundred “scout” bees are the first to venture out from a hive when things start getting cramped. These scouts return to the hive and do a little dance for other worker bees to convey the locations of the potential new sites. Eventually, the hive settles on one, and then the “daughter queen” who will head the new hive sets out, protected by tens of thousands of loyal workers, representing about half the original hive. A high-quality site is ideally one that’s not too far from the old hive (moving out has to happen quickly), near good sources of pollen and nectar, and well sheltered from the wind, extreme temperatures, and other physical disturbances.

Perhaps it’s for the best the hot dog bees were sucked up and moved to more peaceful terrain, despite nearby Central Park being full of nectar-laden flowers. Times Square itself is a frantic hellscape, which, ironically, got a long-awaited taste of calm today as the bees frightened its occupants into terrified silence.