There's a stratagem that revolts an army ("Mutiny"), and onther one that suppress the revolt. So, if played in couple, and both successful, you can eliminate an army.
Another stratagem could be "Desertion"...
Otherwise, battles in "independent" provinces against "natives" often bring to such elimination, due to the terrible combat power of natives.
There are a lot of possible reasons, and I'm a bit puzzled about that...
On the other hand, probably the death of your general is a consequence of such army annihilation, not the cause! Yes, I think it's the effect of a devastating stratagem!
