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Best guess, if the rest of your kale is ok then it could have been an isolated ground dwelling organism (grub?) that crewed on the roots. My kale has done best when direct seeded into the ground rather than transplanting it. It grows fast from seed and does well in cold conditions.

With Brassicas they can die from root maggots, they destroy the root stem. I just pulled up a Cauliflower (the head was fine) and the roots were pretty much ruined it was so easy to pull up., so I ate it just in time. The plant wilts when the root fails. But your Kale is small, usually it takes time to kill the plant. Maybe some sort of rot?

As for the green cabbage "worms" caterpillars, use BT Bacillus thuringiensis, trade name Thuricide from Bonide can be found at garden nurseries.

I appreciate the feedback!