Vitamin D3 is fat soluble and isn't peed out of the body like b or c is. You can take too much vitamin d over time and it accumulated to a toxic level. Supplements aren't just nothing, they affect things in your body after all, that's the point. Too much of anything can be bad for your body in the end. It's especially possible if someone already had normal vitamin d levels and then was taking that vitamin for a long period as well. The doses of vitamin d I've seen people post here that they're taking are really high often! It is certainly within the realm of possibility that someone who already had normal levels took took much for a period of time and ended up with toxic levels of d3 in them, which would stress organs and such.