What causes an electric motor body to give an electric shock when a person touches it and the ground together?

That condition is called a "ground fault". That's when the insulation of the motor winding has a break somewhere, allowing current to leak to the motor housing.

The motor SHOULD be grounded, protecting the user from electrical shock, but many older motors were not grounded, providing the potential for just this type of condition.

Newer motors are often "double insulated", meaning that not only is the motor winding insulated, the motor housing is also insulated. Often the housing of a double insulated motor is made of a type of plastic.