The incident occurred with 8:02 remaining in the first quarter of Boston’s 95-83 win over the Indiana Pacers at Conseco Fieldhouse. At that time, Pierce picked up his second foul (on a questionable call) and was subbed out of the game.
The Celtics' local television broadcast on Comcast SportsNet doesn't show Pierce tossing anything into the stands, including his headband. A feed from FoxSports Indiana appears to show Pierce throwing his gum into the stands as he passes the scorer's table, which could explain the fine. The NBA could not confirm whether the fine was related to Pierce's gum.
This is the second fine this week incurred by a Celtics player. Shaquille O'Neal was fined $35,000 Monday for public criticism of officials for a rant following Boston's loss to Orlando on Christmas Day.