how can solid mob die as an etheral one?

[Gnu]

Zukt brutally guts Seraph!
Seraph looks down in terror as his intestines spill out!

<851hp
168ma 228mv>
Sita utters the words, 'yufzxuie'
Seraph has been banished to the astral plane!
You receive 3148713 experience points as your share.
Everything Seraph was carrying suddenly drops to the ground.
Seraph breaks free of the tentacles!

[12345]

From what I can tell, there's an actual flag that allows builders to make mobs like that. There's at least one area filled with mobs like this. Mostly it just makes autoloot useless.