The way in which Iago addresses Brabantio stands completely in contrast to Roderigo's measured language, addressing Brabantio respectfully as "Signior" even in his initial yell to catch Brabantio's attention. Roderigo, we may infer, knows (or suspects) from the outset that Brabantio will recognize his voice. Iago, on the other hand, uses Brabantio's name without an honorific title ("what, ho, Brabantio!") and repeatedly yells "thieves! thieves! thieves!" in such a way as to alarm him.
Later, the language Iago uses to describe the marriage of Desdemona to Othello is very coarse: "an old black ram / Is topping your white ewe." Roderigo then attempts to temper this: "Most reverend signior, do you know my voice?" He is seemingly attempting to calm Brabantio by indicating that he, Roderigo, is a person known to him; when Brabantio declares, "the worser welcome" to this news, Roderigo continues his efforts at placating him: "Sir, sir, sir," "Patience, good sir."
Iago, repeatedly returning to coarse sexual imagery ("you'll have your daughter covered with a Barbary horse") and cursing ("'zounds, sir") is rightly described by Brabantio as a "profane wretch." Probably, Iago feels free to address Brabantio in this way because, unlike Roderigo, he is not already known to Brabantio and does not feel the need to make any particular impression.