I think it is probably for the blood pools that Larian is known for.
Well Larian has said that they are going to push the Mature Rating as far as they can in every way they can, right up to the point that the "ratings board gets annoying". That suggests alot more then just pools of blood or body horror even.
I am a huge Realms Sage as it were and I while you can easily run a PG campaign in FR if you want, if you dig down deeper, its the most explicit and adult settings in D&D. Rape, Magic Rape, Abortion, attempted Infanticide, genocide, magic war crimes, orgies, brothels, Brothel Temples, Graphics scenes of torture and mutiliation, sex with hudeous demons, three ways with shapeshifting Dragons, Gods of Lust/Joy/BDSM in plenty, Serial Killer Gods, Brain Eating horrors, and soooo much more are all things I've run into in FR novels and FR source Books.
