My theory, and only a theory, is that magnesium stearate is added as a lubricant to tablets and capsules; and it can build up on the intestinal walls. That does not make it toxic; but a concern to me.
Common lubricants for such products are magnesium stearate, potassium stearate, talc, stearic acid, sodium lauryl sulphate, and paraffin.
I don't try to avoid them; except SLS. I just try to minimize the use of products containing them. I lean toward powders.