Excitation of a resonant cavity
It is called magnetic coupling because the field that is present at the slot into the waveguide is primarily via the magnetic field.  There may be some energy coupled into the resonator by the E-field, but it will be significantly less than energy coupled by the magnetic field.
Look at the heading for the bottom two pictures.  There are loops in the bottom two pictures because that's about how you magnetically couple a coaxial cable to a resonator, it's no longer about coupling a resonator to a waveguide.
Look at the two pictures on the bottom, that show that the loop is connected to the center conductor and shield of the coax.