SkShapeNode physicsbody weird behaviour - swift

Question

I created a subclass of SKNode:
class SquareDrop: SKNode {
init(image: SKSpriteNode) {
super.init()
self.setScale(0.3
//Set the starting position of the node
self.position = CGPoint(x: 0.5, y: UIScreen.main.bounds.height/2)
//Apply a physics body to the node
self.physicsBody = SKPhysicsBody(texture: SKTexture(imageNamed: "square"), size: CGSize(width: image.size.width * 0.3, height: image.size.height * 0.3))
self.addChild(image)
}
I created an instance in GameScene class and I set up it:
func spawnRain()
squareDrop = SquareDrop(image: SKSpriteNode(imageNamed: "square"))
squareDrop?.physicsBody?.linearDamping = 5
squareDrop?.physicsBody?.restitution = 0
self.addChild(squareDrop!)
}
It is the result:
It works like aspected, but instead of using an image I wanted to draw a square:
func spawnRain() {
//Here I did not use the class that I had created previously
let shape = SKShapeNode()
shape.path = UIBezierPath(roundedRect: CGRect(x: 0, y: 0, width: 100, height: 100), cornerRadius: 5).cgPath
shape.position = CGPoint(x: frame.midX - shape.frame.width/2 , y: UIScreen.main.bounds.height/2)
shape.fillColor = UIColor.red
shape.physicsBody = SKPhysicsBody(rectangleOf: CGSize(width: 100, height: 100))
addChild(shape)
}
It doesn't seem like I aspected (the squares that I created have a strange behaviour). Any hints?
I am sorry for the question, but I am a noob of SpriteKit.