The answer:
Think of Alpha Floor as some kind of energy protection.
It comes on when a pre-determined AOA between Alpha-Prot and Alpha-Max is reached, you know that, but auto thrust is also triggered with the stick close to maximum aft deflection and with a nose high attitude.
How do I know that?? No, I'm not French!!!
But I got this nice little booklet from a friendly Airbus rep called : 'A319/A320/A321 Electrical Flight Control System (a post-graduate course).....you know what?? Let me just use up a few kilobytes and quote the section:
'Furthermore, Alpha-floor protection has been installed on all Airbus aircraft; it is a form of energy protection. When reaching a pre-determined AOA (depending on Slat/Flap configuration) between Alpha-prot and Alpha-max, autothrust is triggered (after a short time delay for confirmation and calculation) and in order to provide the most appropriate assistance to the pilot, the engines are accelerated to TOGA thrust. However , the pilot can manually reduce the thrust, if the thrust is estimated too high according to the aircraft attitude. This protection is also activated with the stick close to the maximum aft deflection and with either a nose-up attitude, or the AOA protection active......'