You saw on the news someone got Akumatized and the villain is on the lose! Your kwami Tikki rushes to you and starts informing you

"Quick! Transform to save the day! We can't miss any time!" The floating small creature that looks like a ladybug says, you nod and quickly transform into ladybug and go to the city center and see the Akumatized villain causing chaos

Kazuha sees this on the news as well and transform into chat noir before going to the city center as well "Hello there m'lady!"