Mike in Seattle said:Might not another walk in the rain, long enough to evenly dampen the entire hat, even out the first shower's rain bumps?
I had a hat "bump" like this once in a light rain. I think it's because it was made of cheap wool felt, which was shrinking on contact with the rain but only in the small spots where it got wet. A thorough even soak in rain will just make the whole hat shrink down a few sizes, which would be bad. I suspect something similar is going on here.
On a side note, I avoid wool felt hats now. lol