Sebastian and Yuri have been in that position for some time. Yuri wanted to speak at those times, but she was dominated by his nervousness which made him unable to speak.
The second time, Yuri slowly removed Sebastian's hand but Sebastian tightened his hug with Yuri. But, Yuri spoke up and said "Sir What can I do for you?". "What are you talking about? Are you asking me if I want a new girl to make out with?" "No si-", but Sebastian interrupted his answer. At that moment Sebastian can't control his pheromones due to his anger toward Yuri, and that makes Yuri feels so weak.
Sebastian immediately saw Yuri's rapid weakening that caused him to stop releasing his pheromones. Gradually, Sebastian let go of Yuri in his embrace, which caused him to breathe easily and again to stabilize Yuri's condition.