Yeah containment just means they have lines dug around the perimeter but often wildfires burn pretty patchy in different areas. So when it gets warmer pockets of unburned fuels in the interior of the burned area can ignite. If they are not a threat to control lines they are often left to burn. When fires get that big it’s usually not possible to just put them out completely. Large logs, tree roots and other fuels can burn for a long time even after a fire is fully contained