A300 yaw damper
 

I was reading about the AA A300-600 tragedy at Jamaica Bay, NY and it reminded me of an incident I was involved in a few years ago. I was on an AA A300 flight from Chicago to Miami. A short while into the cruise the aircraft began to 'waddle'. The motion became quite pronounced and a few of the pax around me became quite uncomfortable and, we heard later, a flight attendant actually threw up. I suspected that there was a problem with the yaw damper and, sure enough, the captain announced that there was a glitch in the computer which was effecting the yaw damper and that the crew would hand fly the aircraft down to Miami. A short while later the yawing motion stopped. My question is, with so much redundancy built into the computer system on the Airbus, how can this happen? As a layman it seems to me that several 'layers' of back-up would have to fail before a flight control surface would be effected. I would be grateful for any explanation from those of you in the know. Thanks,

Del