How can friction cause motion?
This is a common misconception that friction opposes motion.
Actually the true statement is that friction opposes relative motion between two surfaces which are in direct contact.
Hence in order to oppose relative motion of one body friction may provide motion to another body.
We can consider a very simple example that suppose we have a pencil which is kept on a book on a table, now slowly using your hand slide the book gently on the table you may find it quite difficult to slide the book on the table because friction is opposing the movement of the book on the table but as you were pushing the book gently then the pencil kept on the book was also moving along with the book but you didn't put any effort to push the pencil.
Then who did push the pencil yes it is friction which provided motion to the pencil in order to oppose motion of the book.
In this way friction may provide motion to one body in order to stop some other body.