why does Iago leave Roderigo before Barbantio joins them on the street ?

Iago is a two faced rat. He must maintain the illusion of being loyal to different people at the same time. Above all he must appear loyal to Othello while working to destroy him through manipulating Cassio. Iago can't be seen with Roderigo so that Brabantio will not know that Iago has any part in bringing the news of Desdemona's elopement.