kettle boils over?

As Stay cool said: Don't overfill your kettle!
You should leave at least one inch, 2.5cm, of free space between the water and the sprout. A little lower is often advisable.
Kettles often have some kind of "Fill mark". Never fill over that mark if there is one. Read the instructions, they'll tell you to what level you should fill. Don't exceede that level.

Electro
June 2015

Yes - you have too much water in the kettle.

When the electricity is turned off the heating element still has heat energy - and is hotter than the boiling water - what you experience is physics.

For the water to stop boiling instantly would be magic.

Stay cool...
June 2015