Once you let yourself into the kendo world, you will have to suffer from this. You have to lift your left heel all the time.
And because you do not know why, you end up with lifting your left heel too much and get told off. It is not fair!
As I always say, there are as many theories as a number of kendoists in the world. This is one of the theories out there.
If you walk backwards naturally, analyze how your feet, toes and heels are moving.
You probably lift your feet first. To do so, you have to move the left knee. And move the entire left foot back. Then the toe lands first. Is this what happened?
When people walk backwards, it is the toes that land before the heels.
Since we have the left foot behind in kendo already, we do not have to move the left foot like we normally do. We just keep it as it has just landed.
All we have to do from there to go back is to move it further back without bending the left knee by pushing the body by the right toe.
We want to keep the movement as small as possible because we can move faster and it is hard for your oppoent to tell what you are intending to do.
Because it is natural and easier for us to go backwards if our left heel is up all the time since it is already behind the right foot, we keep the left knee up.
Plus, you do not want to trip over when you go back.
Anyways, you want to keep the left heel up but not too much. Just a little bit so we are ready to move back if necessary.