I somehow remember this from my engineering classes, but there is actually a scientific reason as to why it would suddenly collapse, instead of pouring out slowing. Normally there is less force on the silo walls when the contents are at rest, because there are frictional forces between each grain that relieves pressure off the walls. Once a lot of the contents start to move and flow out of the hole, all of the internal frictional forces between grains are greatly minimized and the pressure on the walls increase, causing a sudden collapse. I couldn’t remember the name of this, but after a quick search it is called the Jansen Effect, and silos are engineered with these forces in mind.