Why do some meteorites break into many pieces?
As a meteoroid plunges into thicker air, the pressure on its front builds rapidly. If that pressure exceeds the rock's strength, often weakened by old cracks, it shatters and the pieces fall as a meteorite shower.
A meteoroid arriving from space is moving at many kilometres per second. As it descends into denser layers of the atmosphere, the air piling up in front of it presses harder and harder: this ram pressure rises steeply with both speed and air density. At some point it can exceed what the rock can withstand, and the meteoroid breaks apart, often at heights of roughly 20 to 50 km.
Most space rocks are much weaker than a solid lump of the same material would be. Asteroids have been battered by collisions for billions of years, so the fragments that become meteoroids are often riddled with cracks and shock veins. Many meteorites are breccias, rocks made of broken pieces cemented together, which can come apart along old seams. Some asteroids are even loose piles of rubble. Iron meteoroids are much stronger, but even they can split.
Fragmentation often happens in stages, each producing a flare of light and an extra boom. New surfaces are briefly heated, so pieces that broke off late may have thinner crust on some faces. The fragments then spread out along the flight path, landing in a long oval strewn field with smaller stones up-range and larger ones down-range.
Some falls have produced astonishing numbers of pieces. When the Holbrook meteorite fell in Arizona in 1912, an estimated 14,000 or so stones rained down, many smaller than a pea. The L'Aigle fall in France in 1803 produced around 3,000 stones, and the Sikhote-Alin iron of 1947 scattered tonnes of metal across Russian forest. The Chelyabinsk asteroid of 2013 also disintegrated, dropping thousands of fragments, from tiny grains to a piece of more than half a tonne.