[OHBBREF]

did the player catch the ball with one foot on the floor then step in the back court, Clearly violation.
if the player was airborne caught the ball then the player has the right to land and can not be called for a back court violation if one foot lands in either court in any order.

But the throw in ends when the ball is legally touched on the court which would be when the ball is touched and the players foot is on the floor.

The exception here is that the airborne player is allowed to land getting both feet down safely.

I do agree with Snaqs it is the fact that once the ball is legally touched on the floor the throw in is over then the violation occurs.