Pretender stabbed to death?

[ArkhanTheBlack]

My pretender got suddently a throwing dagger into his heart out of nowhere. I guess it was some kind of assassination attempt, but I thought assassinations result in a little combat with a few body guards. My pretender could easily beat an entire army on its own, so it's a bit odd to loose him by a thrown dagger.
How does it happen and what can I do to avoid it? And how can I do the same against an opponent?