This is one of the functions of the particle に; to mark a reason/cause of some action. You can translate that part using "due to", "because of", "on seeing", etc.
デジタル大辞泉 lists this as the seventh meaning of に:
This page explains this usage as the 13th meaning of に:
In your sentence, the chairperson is asking to someone not mentioned in this sentence (presumably, to the speaker on the stage). That person could also be marked with に, but is simply omitted here.
Some verbs take に so often (like ～にびっくりする and ～にあきれる in the link above) that it's perhaps best to just memorize the verb-particle pairs, but that's not the case with your sentence.