Miraculous: Contingency began because, just before she went to talk to Marinette in Chat Blanc, the Bunnyx from the Prime timeline attempted to create a “Contingency Timeline” by going back to a few days before Chat Blanc began and creating enough of a disturbance that Papillon decided to create an Akuma.

This Akuma, The Retailer, didn’t exist in the Prime Timeline. When Bunnyx and Ladybug managed to fix Chat Blanc without needing the new timeline, Bunnyx tried to go back and close that time-loop, only to discover that the new timeline had taken on a life of its own, preventing her from influencing it.

The timeline split was not a clean change. Although Chat Blanc didn’t happen, several events that took place afterwards still got carried over, because time travel is a dangerous thing. Time was also slightly damaged by this unintentional split, manifesting in the time that seasons 1-3 took place over stretching out to over a full year.

Some events in history got erased from the Contingency timeline, meaning that the United Heroez don’t exist in Miraculous: Contingency. Therefore Ladybug and her allies are the only superheroes protecting the world.

As Miraculous: Contingency begins, Marinette and most of her friends are nearly sixteen year old. Because she’s been fighting Papillon for over a year, she is much more tired of the fight, leading to her making some different decisions to those which Marinette Prime would make.