I am still perplexed. Even bacterial conversion of CYA to ammonia would tend to make the pH rise, not fall. The only thing that has the pH fall is when chlorine gets used up or consumed -- oxidation of ammonia or other organics is an acidic process. However, if you add a hypochlorite source of chlorine, then that exactly counteracts that and in fact has a small net pH rise due to the excess lye in bleach and chlorinating liquid. If Trichlor or Dichlor were being used, then a pH drop would be expected.

How about your TA level? Has that dropped some as well? An acid would drop both pH and TA.