Why Does My Model Sink Halfway Into the Ground After Adding an Animator Controller?

After adding an animator controller, my model behaves strangely, appearing halfway submerged in the ground.
Without controller


With controller (just a newly created empty one)

I downloaded the model here and added 6 extra bones to make it compatible as a humanoid model (allowing me to use animations from Mixamo.com).


Did I make a mistake somewhere?