In Prisoner of Azkaban, Remus Lupin tells Harry, Ron, and Hermione that werewolves are dangerous only to humans:
‘They couldn’t keep me company as humans, so they kept me company as animals,’ said Lupin. ‘A werewolf is only a danger to people.
Prisoner of Azkaban - page 260 - Bloomsbury - chapter 19, Moony, Wormtail, Padfoot, and Prongs
Yet, after Remus transforms following the events in the Shrieking Shack that led to Peter Pettigrew's identification, Remus's admission that he was a werewolf, and Sirius Black's status as a dog Animagus, while on the way back to the castle, Remus transforms into a werewolf and he and Sirius (in Animagus form) get into a pretty vicious physical fight.
As the werewolf reared, snapping its long jaws, Sirius disappeared from Harry’s side. He had transformed. The enormous, bear-like dog bounded forwards. As the werewolf wrenched itself free of the manacle binding it, the dog seized it about the neck and pulled it backwards, away from Ron and Pettigrew. They were locked, jaw to jaw, claws ripping at each other –
Prisoner of Azkaban - page 279 - Bloomsbury - chapter 20, The Dementor's Kiss
But then, out of the darkness, they heard a yelping, a whining; a dog in pain ...
‘Sirius,’ Harry muttered, staring into the darkness. He had a moment’s indecision, but there was nothing they could do for Ron at the moment, and by the sound of it, Black was in trouble –
Prisoner of Azkaban - page 280 - Bloomsbury - chapter 20, The Dementor's Kiss
I realize Sirius went after Remus first, so Remus probably reacted instinctively and fought back initially. But why would he continue to fight physically after gaining the upper hand, to the point where Sirius was seriously injured? Remus had made the point that when his friends, transfigured as Animagi, were with him while he was a werewolf, it helped to calm Remus and help him tolerate his werewolf transformation.
Why on this particular occasion did Remus attack Sirius so viciously when Sirius was in his dog form? Shouldn't Sirius's presence as a dog have helped to calm Remus rather than incite him?