I'd be curious to know about that rain too - maybe you could set out a small container to collect a sample next time it rains. then test the ph of that to ensure that indeed is the culprit, and not your soil.
Yeah many things can affect soil pH, like perhaps runoff from somewhere?...
I cruised around outside and tested the pH of rain water collected in empty buckets that were chilling out my back door, so it didn't run off my roof or anything. Swear to gods, it was quite acidic the time before last that it rained, and last time it was 8.7 in the same buckets collected from the same spot, still clean and empty.
I was shocked. Getting it as acidic is bad enough, right, but now it VARIES? Acidic when blown in from the west, alkaline when blown in from the south or south-east...
I am investing in good carbon filters and starting to use water that ran through a wetland already. Tested it, and aside from the high silt content when it comes around where I can get it, it's the best untreated water I can find. Better than the rain :/