That depends on what kind of blur is that.
1) If its motion blur, you are shooting in low light with low shutter speed and without a tripod. Your hands are not steady.
2) If its a lens blur:
2a: You are again shooting in low light and its difficult for auto-focus to distinguish between contrasts.
2b: There is something else going on with your lens focusing motor. (I ripped my auto-focus system out of my lens long ago, made my lens lightweight and more reliable).
Note: Though I wouldn't recommend 2b's example. That would cost you your warranty.