The input image is in Rec 709. If the pipeline is set to Rec 2020, a larger space, then all colours are somewhat desaturated (because Rec 2020 is larger). So, Rec709 red (1, 0, 0) may be (0.8, 0.1, 0.1) in Rec 2020, which is like applying attenuation. And since Rec 2020 primaries are also ‘rotated’ with respect to Rec 709, you get the full effect. 
You can find a diagram e.g. here: What is Rec.709 vs Rec.2020 in video? | Digital Camera World