That's because they're the classic places for 'something' to appear. Like the grand piano that plays by itself, the human anatomical model that moves by itself ...

The light went out by itself.

Although there was no wind blowing, the door opened of itself.

It's strange, but the door opened on its own.

Strange to say, the door opened of itself.

She woke up on her own.

He woke up of himself.

To his amazement, the door opened all by itself.

The decayed tooth came out on its own.

The door opened automatically.

A crowd gathered of itself.

Strange to say, the door opened of itself.

The candle went out of itself.

The candle went out of itself.

The door opened automatically.

The tree fell down by itself.

The door opened of itself.

The door locks automatically.

The door shut of its own accord.

The candle went out by itself.