The only thing that makes sense with the time table set by events and info they gived on books and blizzcon:
When bolvar got crowned the death circle broke, why that event? Souls trapped in frostmourne managed to get into the right covenant after the sword broke, bolvar get crowned a few minutes after that, souls trapped inside the sword already are in the shadowlands when this happen, sylvanas suicide after bolvar is crowned and she gets to the maw(she is not evil enought to get there by herself unless they say that those cursed by undeath go to the maw too). So thats the only thing that fits
Edit: another possibility is: Arthas soul is "innocent"(sylvanas says arthas soul is a litle kid) and he got to the maw because frostmourne and/or the helmet cursed his soul, so that broke some kind of seal binding the jailer(supernatural lucifer plot)