NBA coaches have a tendency to not want to officially call a timeout until their team has the ball past halfcourt. This means that the point guard is tasked with dribbling the ball down the court, then stopping to signal for the timeout once he gets to the sidelines. Usually, this happens without incident, because teams kind of stop playing knowing that the timeout is coming anyway.
Hill was rushing to the sidelines to preserve as much of the shot clock as possible, but Heat guard Mario Chalmers stepped in to cut off his path. Hill couldn't slow down and instead barged right into Chalmers for the offensive foul.
Pacers fans might call it cheap on Chalmers' part, but I call it smart. Hill has to know that he can't accelerate there, given that he already has four fouls. Chalmers knew, and made a game-changing play because he did.