Part of Shakespeare is just accepting that some things happen by coincidence to move the story forward. Bianca flinging the handkerchief at Cassio is one example of this. It might not be perfectly plausible, but it is definitely necessary. The handkerchief represents Desdemona's fidelity and Othello seeing it in another woman's hand from the man he thinks Desdemona is having an affair with, solidifies Iago's suggestion of the affair. The handkerchief represents Desdemona's innocence and when he sees it, her possibility of innocence is shattered.