In my understanding, those who practice magic are working with or manipulating energies for an intended purpose. If that is the case, I would imagine this could be viewed much in the way one would see the flow of electricity.
If electricity is flowing through a circuit someone creates for the purpose of lighting a light bulb, and an outside grounded conductor were to come in contact with that circuit, the flow of electricity to the light bulb is disrupted and no longer able to light the bulb.
So it would stand to reason that someone who believes in the practice of magic would believe that touching someone who is performing a magical ritual or spell might disrupt the energy flowing through the mage.