After listening so intently, the audience needs to let off steam. The musicians made a lot of noise and so now the audience wants a chance. So that everyone can enjoy the music without distraction. They are afraid of being kicked out if they distract the musicians. Actually, this is an important point.
Music is a listening experience.
You cannot really enjoy the music if you are constantly distracted by people talking, coughing, or whispering. If you are talking, you cannot listen and you are disturbing others. If you are listening, you are part of the concert. The music - the creative work of a composer 2. The performers - the people who make the music sound and interpret the composer's ideas 3.