Now, let's perform a process of elimination. There is no common danger that threatens those at the scene. You have done nothing to threaten him. Neither has anyone else. He has not been exerting himself. Yet, his blood vessels are pulsing violently and he is breathing heavily. By this process of elimination, we can determine where the fight or flight thing has come from: He has already decided that he is going to fight. (Or, if you are lucky, that he is going to run.)
The adrenal system instantly releases powerful chemicals in a fight or flight state, including epinephrine ("adrenaline"). One side effect of this is tremors, often violent ones, which will usually manifest themselves first in the non-dominant hand, almost immediately thereafter in the dominant hand, and then in the legs, particularly the knees.
If you observe tremors in those locations in a situation that you perceive may turn hostile, go through that process of elimination again. Could the person be simply shivering in the cold? Do you have reason to believe he has Parkinson's disease or some other ailment of which trembling is symptomatic? If not, you know the diagnosis, and you know the first step of treatment—create distance.