When deciding on what action to take when a player kicks, trips, pushes, strikes, charges, jumps at, or generally just tackles an opponent, the referee must consider whether it is careless, reckless, committed using excessive force, or alternatively, none of the above. p. 36 of the 2014/15 FIFA Laws of The Game
On page 117 of the 2014/15 FIFA Laws of The Game:
"Careless" means that the player has shown a lack of attention or consideration when making a challenge or that he acted without precaution.
Only a direct free kick / penalty kick is awarded if the action is careless.
"Reckless" means that the player has acted with complete disregard to the danger to, or consequences for, his opponent.
A direct free kick / penalty kick is awarded if the action is reckless, and the player is also cautioned for unsporting behaviour. p. 123 of the 2014/15 FIFA Laws of The Game
"Using excessive force" means that the player has far exceeded the necessary use of force and is in danger of injuring his opponent.
A direct free kick / penalty kick is awarded if the action is excessively forceful, and the player is sent-off for serious foul play if it is performed whilst challenging for the ball. If not challenging for the ball, the player is sent-off for violent conduct. p. 126-128 of the 2014/15 FIFA Laws of The Game
If a player kicks, trips, pushes, strikes, charges, jumps at, or tackles an opponent in a manner that is not careless, reckless, or excessively forceful no offence has been committed and play continues.
As Sean pointed out, whether or not the action is accidental has no bearing on the decision that should be made.
Judging the particular incident, Boye was pushed by Dempsey first (notice the arm on the chest), which led to Boye being off balance. However, swinging your leg high into an opponent's nose, while you and your opponent are in the air, and you are off balance sounds to me like it is at least reckless, and possibly excessively forceful. I would have no complaint with Boye being either sent-off or cautioned for this challenge.