User blog comment:Zelron/Soul of the Dragon (Why the soul gotta be black?)/@comment-30303790-20121002192452/@comment-3492791-20121002204152

The Ideal Masters didn't soul trap Durnehviir, they trick him.

Durnehviir went to the Soul Cairn of his own accord to ask the Ideal Masters for "the power to raise legions of the [undead]" (which was probably the Soul Tear shout). In return he had to stay until Valerica was dead. Since she was also an immortal, she wouldn't die. Thus they tricked him and he was bound under oath to stay there from a potential eternity.

And it's obviously not impoosible for the Dragonborn to be soul trapped. It's already happened twice!