In Macbeth, how do the witches contribute further to Macbeth's downfall when he revisits them in scene 16?

I assume you are reffering to Act IV, scene i when Macbeth seeks out the witches to get advice and find out more of his fate. The significance here is that Macbeth is seeking the witches out this time. He is taking control of his own terrible fate and the witches acknowledge this, saying "By the pricking of my thumbs, / Something wicked this way comes." Instead of hapless victim, Macbeth is an instrument in his own downfall.

The prophecies of the witches both encourage Macbeth, since he feels that they grant him immortality, but they are paradoxically troubling, too. After all, if cannot be defeated--since every man is born of a woman and forests grow very slowly--why would Banquo's sons be shown as kings? Macbeth chooses to select the information that benefits him and ignore (as much as he can) the rest.

The bottom line here is that it is Macbeth making the decisions. He does not appear to be under the witches' power or spell here and Lady Macbeth is nowhere to be found. This is Macbeth choosing his fate.