I agree that it is unlikely to be a chemical reaction. Instead, I suspect you are nucleating bubbles for the dissolved gasses in the solution (think diet coke and mentos).

When yeast is active, it creates CO₂ as a byproduct. This is how you get bubbles on the top of your solution when you are blooming the yeast. This gas byproduct is also dissolved into the water as well. Letting it sit for that long would give it plenty of time to completely saturate the liquid (or even supersaturate it depending on environmental changes).

When you throw the salt in, those salt crystals act as a nice nucleation point allowing those dissolved gases to form a bubble and leave the liquid phase. I can’t really speak to how salt behaves differently to flour in this regard, but nucleation rate is proportional to the available surface area for nucleation, and salt crystals tend to not clump up nearly as much as flour, making more surface area accessible for nucleation.

I suspect you already know this since you make so much bread, but, I was always taught that introducing an osmotic shock like adding salt directly to your yeast will slow down the rising/proofing process. So, it would be best to add the flour, then the salt on top, then mix them together to help blunt the osmotic stress on your yeast.