I had a similar phenomenon earlier this year, and I'm not sure I ever found an explanation for it. I did raise my TA a little bit, to a little lower than you have it now. That didn't seem to have much effect. The only thing I could think of was that we were getting a lot of leaves, pollen, etc. in the pool and that might contributing to the lowering of the Ph.

For us this lasted about a month or so and then just stabilized. The extra debris in the pool was the only thing I could think of unless some of the big showers was adding acid to the pool, which is possible, but it seemed as though my Ph would drop whether we had rain or not. It has now been a consistent 7.6 for several months.

Riles