Griff had a birthday, and made sure everyone knew it. You can do that when you're the only one at work who knows how to change the sign. He didn't post this picture at reddit, but he did join in to explain when extramediumjohn did.
To be fair, I did it without asking, but it's not like they can fire the guy in charge of the sign.
Not on his birthday, anyway. And after all, the sign doesn't say anything malicious or derogatory. But at least now everyone at work knows the power of the Griff.