Think about the magnetic field relative to the actual coil when it has turned 180° and is upside down - lets call the left side of the coil before it moves A, and the right side the coil before it moves B. Initially, the magnetic field goes from A to B. Once the coil has rotated 180°, the magnetic field now flows from B to A. Since the direction of the field relative to the coil has changed, the induced current must also have changed direction, and this repeats as the coil continues to turn.