If the chlorine level was high due to shocking at 15 ppm FC, then the pH may not be accurate and could falsely read higher than it really is. Nevertheless, even if it were accurate, if the FC drops from 15 to 3, for example, and if 1 ppm of this drop "broke" Combined Chlorine, then the pH would be expected to drop frm 7.5 to 7.04 due to the fact that the process of chlorine breakdown (from sunlight or oxidizing organics or breaking down ammonia/urea or monochloramine) is an acidic process. So the drop in pH is understandable if the FC was dropping along with it.

The above scenario would only have the TA drop from 80 to 70, however. To have the TA drop to 40 would require a lot more acidic processes or additions along with aeration to drive out carbon dioxide. Raindrops can certainly aerate the water, so starting with the above numbers of 7.04 pH and 70 TA and going to 6.9 pH and 40 TA would require the equivalent of 72 fluid ounces of Muriatic Acid in (assumed) 15,000 gallons. If we assume 16x32 feet for the pool area, then 0.5" of rain is around 160 gallons so the required amount of acid in that volume would have a pH of 1.5 while even acid rain doesn't get below 4.5 so the very low TA number you measured is suspect and unexplained.

Richard