@Bjain Thousands of reasons.
Over voltage via any components.
Incorrect wiring.
Placing on a metallic surface when powered.
Careless soldering solder splashes on the board.
Leaving in the anti-static foam whilst powered up.
Over exertion on the board or its components.
The list of things that will let the magic smoke out is as long as your arm and all of them only take a fraction of a second to do the damage.