It's that moment where the goat looks like he's about to charge the other guy that Inigo can't help but move. It's only a single step, since he notices Quatre calming the goat down again a moment later, but he still can't help himself.

Maybe he's a little too focused, ready to jump into the line of fire if the other looks like he's in trouble. Inigo is a natural protector, after all.

But with the goat settled down again for a moment, he just frowns. He doesn't speak, not wanting to draw the attention from the goat towards himself, but he mouths at Quatre 'start walking backwards', hoping the other will understand.

After all, they have to make the animal follow Quatre.