Looking at the featured questions page, there are a pile of really old questions that have new bounties. One even has an accepted answer.
Does this happen every year as an attempt to get all the hats?
P.S. I like the hats.
Yes, it's for hats.
The Gelt hat is for a bounty offered during Hanukkah (December 16 and December 24) and there'll be more offered on Christmas day for the Stockings Hung by the Fire hat.
But not only does the bounty-offerer get a hat, anyone who answers a bounty (and gets an upvote) also gets the Major Award hat!
It does not matter on what site you set up a bounty. You get the Make it rain hat when you award that bounty to someone.
IG-88 was a less well-known bounty hunter, and the hat that bears his name went to users who tried for a bounty, but didn’t win it.