From the BBC article, posted above as source!
Djokovic had been playing well up until the game where he lost serve and had three set points at 5-4 before Carreno Busta fought back from 0-40 down.
However, the world number one also showed a flash of his temper during that ninth game by whacking a ball into an advertising board after the Spaniard brought it back to deuce.
Leon Smith, Great Britain's Davis Cup captain, said he was "surprised" Djokovic did not get a warning for that incident, and felt one might have prevented the controversy which followed.
After Djokovic was defaulted, Smith told BBC Radio 5 Live Sports Extra: "There has to be consistency, if someone hits a ball with that much venom and temper, call a warning.
"The second one can be dangerous and so it proved to be."
In what proved the final game, Djokovic fell and hurt his shoulder, immediately calling for a medical timeout when trailing 0-30. Following treatment, the match resumed with Carreno Busta sealing the game three points later.
It was then that Djokovic hit the ball away, striking the line judge
So, he already did it once, escaped luckily. Not the second time! This seems like pure arrogance and frustration.