Man having dealt with so many wireless issues I already had a shortlist when I read fixed wireless was involved.

Either the radios were misaligned and the rain was reflecting it back towards a stable link just enough to improve throughput.

Or

The rain took a bad link all the way down, failing over to a different link.

Or

The rain/wind was moving an obstruction.

I have about a million of these stories sadly.

"The internet goes down on tuesdays"

Crane.

"The internet goes out in the morning"

Temperature inversion.

“The car won’t start after I buy ice cream”

The old classic, I think it turned out that in hot weather the fuel line vapour locked a few minutes after turning off, and the ice cream was at the back of the store, and took just long enough to walk there and back to trigger the issue.