Well, you've got to kill that time between boarding and taking off somehow, haven't you?
And if you're in the cast of 'The Lion King'... you do this.
"After an amazing day at The Lion King Brisbane season launch announcement, members of our Australian cast felt so elated they decided to sing the 'Circle of Life' prior to take off on their flight back to Sydney!" explains DisneyOnBroadway.
Presumably they quietened down during the in-flight safety video.