If you started with some amount of TA, then a combination of acid addition and aeration would cause the TA to go away. Were you using Trichlor tabs/pucks as your source of chlorine and if so were you using only Borax to raise the pH? If so, then that might explain what happened. When using Trichlor (or any acidic source of chlorine including Dichlor) you should use pH Up / Arm & Hammer Super Washing Soda / Sodium Carbonate as your base to raise pH. This will add carbonates back to the water. When using Trichlor you want the TA to be higher (usually at least 120 ppm) because you need more pH buffering and can achieve a sort of balance between Trichlor acid, carbon dioxide outgassing and sodium carbonate addition.

If you have been using Trichlor, then you should check your Cyanuric Acid (CYA) level as it could be high.

If you haven't been using Trichlor and were using bleach or chlorinating liquid or even Cal-Hypo instead, then were you seeing a frequent rise in pH and adding lots of acid frequently to compensate and keep the pH lower? If so, what was your pH target?

Also, do you have a waterfall, spillover, vanishing edge, fountain, shower or other aeration water feature?

Richard