Magnetizing current in switching converter
That picture is incorrect for anything other than what it is intended for i.e. to demonstrate that you need to reset the flux in an inductor or transformer.

What happens in reality with the circuit shown is that the energy built up (by the rising current) creates a high voltage when the MOSFET turns off and destroys it. They are using this circuit as a "device" to explain the problem.

If you'd shown the full picture and read the subsequent pages you might have made more sense of it: -