What is the reason those powerful toothpastes make gums slough?

Harsh chemicals. Some of the toothpastes out there will, in some people, cause sloughing of the gum. If this happens to you, I would recommend a toothpaste without sodium laurel sulfate, the most common ingredient to cause side effects, although whitening toothpastes can do it because of the whitening ingredient as well!
Too strong. Some toothpaste companies make stronger toothpaste to help remove stains and plaque better but some people might be allergic or sensitive to these products, stop its use but also consult a dentist to make sure it was the toothpaste that cause the gums to become sloughish.
Chemical irritation. These toothpastes may contain powerful chemicals or chemicals that you are sensitive to that would cause the outer surface of your gums to become so irritated that I can slough off. Stop using those products.