Fine, new official explanation!
John Samson has an embarrassaing secret. He suffers from an ailment that one out of every 10,000 trolls suffers from: Cope's Disease. Cope's disease weakens the structure of troll horns, causing them to fall out. Most trolls effected by this are over the age of 40, but John was stricken when he was 18. He was mocked and ridiculed by every trolls he knew, because horns are frequently a source of pride among them. So now John wears Hornpiece for Trolls, an artificial set of horns designed to hide the fact that he has no horns. They look awful, and everyone knows they're fake, but everyone is too polite to say anything. They're the troll equivalent of a toupee.
Now, isn't this supposed to be about the mini's game?