Sometimes it's because you know exactly what you want to say in those situations. You have to think quicker so your mind's initial thought matches what you try to say. Or it could be that the willingness to express the emotion behind arguing makes you more expressive in general. Or that you just really want to say what you have to say. Ever wanted to tell someone something so eagerly that you forget you have a stutter entirely? Just a few possibilities. I've experienced all of them to some extent.