Reported by Nissa Wang here:
https://groups.google.com/forum/?hl=en#!topic/ns-3-users/RJvwo-Uwx7s

The Bounds attribute default value is:
    .AddAttribute ("Bounds",
                   "Bounds of the area to cruise.",
                   RectangleValue (Rectangle (0.0, 0.0, 100.0, 100.0)),
                   MakeRectangleAccessor (&RandomWalk2dMobilityModel::m_bounds),
                   MakeRectangleChecker ())

However, the rectangle constructor is:
Rectangle::Rectangle (double _xMin, double _xMax,
                      double _yMin, double _yMax)

As a consequence, the default "Bounds" attribute is a *point* (and the node will stand still).