Its to highlight a new feature, instead of using the slide to unlock, when you have a notification on the lockscreen, you can slide the app icon and slide to view the notification, so it goes straight to the app giving the notification.
Once you've done this one, they won't wiggle again because they've taught you what to do.
I only found this out because I also wondered why it was wiggling, pressed it while it was wiggling and lo and behold the slide to view appeared. I then realised what the wiggle was for, pretty clever of apple really.