Why do some players hit a ball across the net after he end of a game?
Very simple: the rule has been set (even tightened recently) that after an out call, in case of challenge and call overturn, the unlucky player must have a "clean play" on the ball, in order for an umpire to call replay. Even if the unlucky player did have a play but missed,the umpire's judgement can still be that the call did not hinder his play and rule a point against him. This judgement call often sparks controversies. So, to avoid controversy players learned to hit "one extra shot" after each out call (when close). We had an example of such judgement that derailed Tsi's matchpoint in 3rd set (and that was just a very bad call by the linesman), fortunately, Tsi regrouped and won TB despite losing a game due to a bad call.
I've been a vocal critic of these judgements that were created as a result of hawkeye. Players have already adjusted to play an extra dead shot as a result. Why can't linesmen also adjust and
do not call close out until the player plays the final shot so as to not interfere with the play? Essentially, make all out calls "late". Such simple adjustment would have spared the umpire the difficult judgement decision and would have spared Tsi his frustration. But why do silly linemen keep shouting their out immediately as the ball hits the line, and interfering the play? It's not that difficult to understand why all out calls should be late in hawkeye era, and only a little bit of practice to adjust the calls.