It's some weird tradition they've started doing that I either never noticed before or it's new. People used to bite gold coins to prove they were real gold since gold is soft enough to leave a tooth mark, but I don't know if that's related to this or what. You couldn't do that with silver or bronze.
I always associated it with Underdog (the superhero) as he bites the coins he's tipped as a shoe shine to test their authenticity, quite often with disappointing results.
David Wallechinsky, President of the International Society of Olympic Historians told CNN in 2012, “It’s become an obsession with the photographers. I think they look at it as an iconic shot, as something that you can probably sell. I don’t think it’s something the athletes would probably do on their own.”
i know china does this with gold tender, i'm not sure if this is just chinese, or if this is more global, but when doing transactions it's customary to take a nips to the gold to make sure it's solid gold all the way through so eventually it ends up with a number of little bite marks in it.