AN A319 ON A NON PRECISION APCH BEGAN AN UNCOMMANDED DSCNT PRIOR TO THE INITIAL APCH FIX DSCNT POINT.

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WHILE UNDER CTL OF APCH; WE WERE VECTORED ONTO THE LOC FOR THE LOC RWY 27 APCH. ONCE CLRED FOR THE APCH; ALL ACFT INDICATIONS WERE NORMAL UNTIL SHORTLY JUST PRIOR TO 'REEBO' INTXN. APPROX 1 MI E OF 'REEBO' INTXN (WHICH HAS AN INTERMEDIATE ALT OF 2000 FT) OUR ACFT'S AUTOMATION MALFUNCTIONED; AND WITH AUTOPLT ENGAGED IT BEGAN A SPONTANEOUS DSCNT. WE DISCONNECTED THE AUTOPLT TO REGAIN ALT; BUT SAW THAT OUR ACFT HAD INADVERTENTLY DSNDED TO APPROX 1600 FT MSL; 400 FT LOWER THAN THE PUBLISHED ALT TO REEBO. WE RETURNED THE ACFT TO THE PROFILE FOR THE LOC APCH MANUALLY AND THE REST OF OUR DSCNT AND LNDG WAS UNEVENTFUL. AT NO TIME DID OUR CTLRS MENTION THE MOMENTARY DEV; AND AT NO TIME DID WE PRESENT A THREAT TO TFC OR TERRAIN. WX WAS VFR AND WE CORRECTED IMMEDIATELY. THIS EXPERIENCE HAS CAUSED ME TO BE MUCH MORE VIGILANT OF THE AIRBUS'S AUTOMATION; PARTICULARLY DURING NON PRECISION APCHS. I WILL CONTINUE TO AGGRESSIVELY MONITOR ITS AUTOMATION AND MORE IMMEDIATELY INTERVENE WHEN ENCOUNTERING DEVS IN THE FUTURE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE PLT RPTED THAT HE HAS HEARD OF SEVERAL A319 UNCOMMANDED DSCNTS. THIS COMPANY'S NON PRECISION APCH PROC IS TO SET A FLT PATH ANGLE (FPA) PRIOR TO THE DSCNT POINT; AND AT .3 MI PRIOR; ACTIVATE THE DSCNT THEN SET THE MISSED APCH ALT IN THE FCU ALT WINDOW. IN NO CASE SHOULD THE ACFT BEGIN A DSCNT OUT OF THE FCU ALT SET UNLESS EITHER FPA OR OPEN DSCNT ARE SELECTED. WHEN ASKED IF THE CREW HAS SELECTED AN RNAV BACK UP IN THE MCDU APCH PAGE; THE RPTR STATED NO; ONLY THE LOC RWY 27. THE RNAV WOULD HAVE ALLOWED 1800 FT AT THE OM AND WOULD HAVE AUTOMATICALLY BEGUN A DSCNT.