Them and others. Liu Lang and his followers weren't just chilling on their giant turtle, they were actively exploring the world. Even after Liu's death, when the Wandering Isle community became passive for the most part, there were still Pandaren like Chen, affected by the "wonderlust", who prefered to leave their home for the sake of exploring.
I'm not talking about what's happening these days. The DK starting experience is set in the past, back when the LK was very much alive. My point is it's not impossible for some wandering Pandaren explorers, who stopped by to help the Argent Dawn, to be slain and raised back then. Blizzard has woven goblin and Worgen DK's into that exact time period in the same way.