When you write software, you should only change a default value for a good reason. This was...not that.
I agree that the outrage can sometimes seem out of proportion to the change itself, but I can also understand why people who write software in general would be offended by the silliness of the whole episode.
I read the announcement from Oct. 1 [1] and it doesn't have any explanation outside of a link to a Software Freedom Conservancy [2] (the folks now maintaining Git)
A lot of people here are assuming virtue signaling, but it could just as easily be "a majority number of our staff was behind this change". Unless GitHub has stated the why somewhere (I spent ~5m googling to no avail) we simply don't know.
I agree that the outrage can sometimes seem out of proportion to the change itself, but I can also understand why people who write software in general would be offended by the silliness of the whole episode.