Divorce your ego from the equation, particularly when there is no evidence of an imminent assault.
Often, the person asking, "Why?" is simply one who subscribes to the "Question Authority" School of Thought. Unfortunately, they don't all adhere to the "When Authority Answers, Listen" School of Diplomacy. Here's where a little verbal judo can go a long way.
If you have the luxury of time to answer the person's questions, then do so. If he persists in his refusal to cooperate, the fact that you attempted to engage him in conversation will work to your favor in any subsequent review of the incident.
If the situation is one wherein you suspect it may be a matter of a personality clash, ask for only specific units to respond. This will keep the officers involved limited to those you want on scene (people adept at talking agitated people down) while preventing needless frustrations from others (the cop whose Indian name is He Who Pisses Off Many).
At all times, continually evaluate what the person is truly angling for. Is he grandstanding for the benefit of onlookers? Attempt to remove his audience. Is he stalling for time to orchestrate an attack or escape? Take a protective posture, and possibly get units rolling. Is he even in his right mind? The situation may call for intervention that is beyond your skill-set.