Why do crowd crushes happen?

On 29th October 2022, 20,000 people were celebrating Halloween in the Itaewon neighbourhood of Seoul in South Korea. One particularly narrow street became very overcrowded, and a huge crush ensued, leading to the deaths of 153 people, with many more injured. A month later, we still don’t know exactly what caused the crush. 

One of the theories out there is that a rumour spread in the crowd, leading them to believe that a celebrity was in a nearby bar. But a lot of blame has been apportioned to the authorities for poor planning and a slow response to events. A l...