The rabona does not occure randomly. You can do it by pressing some buttons. Only skilled players can do it.
YouTube - FIFA 09: Rabona tutorial
Stand facing the goal. Press the RIGHT ANALOGUE(ps3) towards your players weaker foot(Ronaldos weaker foot is his left, so you press left). Then you press the lob button(ps3: square), and immidietly tap forward on the left analogue(ps3). Try it.