The comboing onto Moon Luring Frost used up many of the skills Boundless Sea had available. During a combo, both sides needed to pay attention to which skills were used. The side performing the combo wanted to leave as few openings as possible for his opponent to take advantage of. The side being comboed wanted to look for opportunities to break out of the combo.
Fang Rui didn't get so carried away that he would leave any huge holes for himself after finishing his combo. However, because he had chained together so many skills, he didn't have many skills left to use and his options were rather limited. If Du Ming counterattacked immediately, Fang Rui would be in greater danger.
Fang Rui clearly felt this danger.
Such a fast counterattack couldn't have been out of impulse. Du Ming had calmly calculated when exactly his counterattack would be most effective.
Du Ming was playing like he was on fire.