Going to link my post when my Muscovy hen attacked her mate (Silver Appleyard drake) when she was broody...Warning ⚠️ graphic photos in the post.
Thread 'Daffy brutally attacks Daisy (Warning: Graphic Pictures)'...
That looks too low for it to be due to mating... unfortunately I can't offer any other ideas as to why she has that injury. Something could have grabbed her, it could have been a squabble within pecking order, she could have gotten too close to a hen that's broody. Muscovy's while broody can be...