So, I have never actually made a gravatar account or changed my gravatar. I've always used the default icon I got when I created my SO account, because I liked it and didn't want to change it.
However, when I logged in today to SO it had changed! Which is strange, because I did nothing to change it. It is even more odd because it has not changed on any of the other SE sites I participate in--on meta, EL&U, and Area 51, it is still the same image as it was when I created my account. It looks just fine.
The only thing I can think of that happened recently is that I gained over 1k rep on SO yesterday. I don't see why that would have changed my gravatar, but it's the only thing that's recent and different from my other SE accounts.
Does anyone have any idea what happened?