How do some artificial sweeteners cause diabetes?

They don't directly. I suppose if a person is dilutional enough to think by avoiding sugar (with such sweeteners) they may feel they can eat all they want. They then get overweight & adult onset diabetes is a potential outcome.

Artificial sweeteners do not cause diabetes directly, but by altering the taste sensation and caloric association, artificial sweeteners often prompt increased food intake that include carbohydrates, weight gain and increased risk of diabetes. For healthy diet: http://www.choosemyplate.gov/.