Why does toothpaste takes the skin off mouth?

Change toothpaste? Certain people are sensitive to the ingredients in some toothpastes. You could be one of these people. The solution may be as simple as changing to another toothpaste. There could also be a more serious problem, so if it lasts for more than a week you should talk with your dentist or hygienist.
It should not. Possible allergic reaction. Brush without toothpaste until mouth heals, then start using toothpaste again, but only a pea-sized portion each time. If sloughing begins again try a different brand. And report this to your Dentist immediately.
It doesnt. Toothpaste doesn't take skin off your mouth. Maybe you're brushing so hard to cause the gingiva to slough off.