DU Lawyers: What is "probable cause" in relation to the AZ law?

[Ratty]

What could legally be a reason for a cop to demand somebody's birth certificate or citizenship papers and then haul them down to the station if they aren't carrying them? What else could there possibly be, aside from race, that would rise to the level of probable cause? Do anonymous complaints qualify? If they are working someplace that has a history of hiring illegal immigrants? This is a serious question.