Perhaps the PF selected reverse thrust a few moments before the 'weight-on-wheels' microswitch changed from flight to ground? If so there can be insufficient time for the ECAM to illuminate before the conditions for the caution are no longer met and the result can be either an abbreviated Single Chime, an odd Master Caution p/b illumination (such as you describe), or both.
There is no danger if this early reverse thrust selection is inadvertently made (hence it is only a Master Caution and not a Master Warning), but I see it happen on occasion and have caused it myself.
Usually it occurs when pilots are "trigger happy" (gusty, bumpy conditions or when landing on short runways) and want to get the aircraft down and stopped quickly, or when pilots manage to really 'grease' it on. Such a good landing even the aircraft is confused!
I can't explain the precise reason why it occurs but after many years of flying the beasties I have just put in the "it's just an Airbus thing" basket!