That was the explanation I was given for burning their dead here, at least for the Andrastians. Spirits watch us, press close to the Veil out of curiosity. When so many die in a place then the spirits can come through and act out what they saw before.

[Given what Thranduil's said, it slots into place but her reading on the Fade was only ever enough to understand how it shaped the history, the politics. It wasn't something she thought she needed but maybe she's neglected it as she's made the efforts to fit in elsewhere.]

They possess the corpse of the deceased. Pick up their weapons. Stand up and fight. It's all they've seen.