Why does my SKShapeNode pass through an object, pausing briefly beforehand?

Question

The code below when run on a device that’s flat on the table shows a red circle (the 'ball') and a line.

When I gently tip the device the circle moves towards the line. When it reaches the line, it stops which is the expected and desired behaviour.

BUT if I keep tipping, the circle moves through the line. I’ve been trying for 2 days to work out why grrr I just want the circle to hit the line and not go through it.

Any ideas? Anything to do with the line being an 'edge'?

Cheers, Andy

import SpriteKit
import CoreMotion

class GameScene: SKScene, SKPhysicsContactDelegate {   
    var ball : SKShapeNode!
    var line : SKShapeNode!
    var motionManager = CMMotionManager()
    var destX : CGFloat!
    var destY : CGFloat!
    let ballRadius = CGFloat(20.0)

override func didMoveToView(view: SKView) {
    backgroundColor = SKColor.whiteColor()
    physicsWorld.gravity = CGVector(dx: 0, dy: 0)
    physicsWorld.contactDelegate = self

    ball = SKShapeNode(circleOfRadius: ballRadius)
    ball.fillColor = UIColor.redColor()

    ball.position = CGPoint(x: frame.size.width/4, y: frame.size.height / 2)
    destX = frame.size.width/4
    destY = frame.size.height / 2
    ball.physicsBody = SKPhysicsBody(circleOfRadius: ballRadius)
    addChild(ball)

    line = SKShapeNode()
    var lineStart = CGPoint(x: frame.size.width/2, y: 0)
    var lineEnd = CGPoint(x: frame.size.width/2, y: frame.size.height)
    var linePoints = [lineStart, lineEnd]
    var linePos : CGMutablePathRef = CGPathCreateMutable()
    CGPathAddLines(linePos, nil, linePoints, 2)
    line.path = linePos
    line.strokeColor = UIColor.blackColor()
    line.physicsBody = SKPhysicsBody(edgeFromPoint: lineStart, toPoint: lineEnd)
    addChild(line)
}

override func update(currentTime: NSTimeInterval) {
    moveBall()
}

func moveBall() {       
    motionManager.startAccelerometerUpdatesToQueue(
    NSOperationQueue.currentQueue(), withHandler: {
        data, error in
        var currentX = self.ball.position.x
        var currentY = self.ball.position.y
        self.destY = currentY - CGFloat(data.acceleration.x * 100)
        self.destX = currentX + CGFloat(data.acceleration.y * 100)
        }
    )
    ball.position = CGPoint(x: destX, y: destY)
    }
}


import UIKit
import SpriteKit

class GameViewController: UIViewController {

override func viewDidLoad() {
    super.viewDidLoad()
    let skView = self.view as! SKView
    let scene = GameScene(size: CGSizeMake(
    skView.bounds.size.width, skView.bounds.size.height))
    skView.showsFPS = true
    skView.showsNodeCount = true
    skView.ignoresSiblingOrder = true
    scene.scaleMode = .AspectFill
    skView.presentScene(scene)
}

override func prefersStatusBarHidden() -> Bool {
    return true
}
}