Why outlets spark when unplugging - EMF & Inductors

Simply put the magnetic field collapses when you disconnect the circuit. The change in the magnetic field around the coil induces the current. 
Going more deep into the topic a magnetic field can collapse really fast which is why coils can create high voltage when you end the current flow suddenly like in car ignitions. Car ignitions are a little different because they have two coils, first coil is 12v and is the one who is disconnected, the second coil is inside the first coil and it receives the collapsing magnetic field and sends it to the spark plug. 
In the demo of this video you have basically a huge single coil. A point I want to bring up to not confuse is that the speed at which a magnetic field collapses is deeply related to the number of turns of the coil. Just like she said the bigger the coil is like the more momentum or something like that 🙃.