I see it like this: people were getting enthusiastic about being at his presence, thus all the ovation, and Pope just joined them in his own way, just making his small “uhu, uhu”, which made people even more enthusiastic. This way he demonstrated, he is with them and he enjoys himself.
When the priest came to him and said something, Pope jokingly revealed what he had just been told, ie. that “we (Pope and audience) will destroy the programme”.
I have seen similar events with him, for instance when he was talking to students in Poland, he liked to show he is united with people, he was joking and laughing with them.