Questions about Inductive Kickback and what causes an inductor to discharge?
When you send current through the inductor, the inductor will use the energy in that current to maintain a magnetic field. When the power source that was supplying the current through the inductor dissapears, the inductor tries to keep the current flowing through it the same and it does so using the energy stored in the magnetic field.

The inductor uses the energy in the magnetic field (which corresponds with the magnetic field collapsing as its energy is consumed) to maintain the current as close as possible to the same current levels as when the current source disappeared.

The inductor does this by becoming a power source using the energy stored in the magnetic field to produce a voltage as high as necessary to push that level of current through any obstacles so that the current flowing through the inductor is the same. If the easiest path is across the air gap of the open contacts of a mechanical switch or relay, the voltage required to spark across that air gap will be very high so that's the voltage that will be produced. That requires a lot of energy so the current will collapse to zero quickly.

If the easiest path is something like a diode then then the voltage (i.e. the diode's forward voltage drop) required to push that current won't need to be so high and the energy stored in the magnetic field lasts longer so the current drops more slowly.