Why do shotgun BBS spread when shot?

Unlike bullets, shotgun pellets do not spin when fired.

The rifling in a rifle or handgun barrel spins the bullet as it travels down the barrel, giving the bullet stability in flight.

Shotguns are "smooth-bore" - they have no rifling inside the barrel, just a smooth tube. Because of this, the pellets do not spin, and thus fly with a certain degree of randomness.

To further increase the "spread", lead shot gets banged up against the inside of the barrel and other pellets while going down the barrel, making the pellets non-uniform or out-of-round, further reducing their accuracy and increasing spread.

Many shotguns have interchangeable "choke tubes" to allow the shooter to control how little or how much the shot spreads out.

However, shotguns do not "spread" as much as in movies. A normal-length (18+ inch) shotgun will produce a spread no larger than a cereal bowl from normal "combat" distances.