From the Great Deep to the Great Deep...

But when that moan had past for evermore,
The stillness of the dead world's winter dawn
Amazed him, and he groaned, 'The King is gone.'
And therewithal came on him the weird rhyme,
'From the great deep to the great deep he goes.'

from Idylls of the King, The Passing of Arthur

From the Great Deep to the Great Deep they went, washed away, snatched, swept away by the foreign tsunami, never before seen or heard in these parts...but it came unbidden, unannounced, the wrath of Poseidon, the rage of Varuna

How do these things happen? and why?