Lemon juice and kale/green leaf lettuce salad produces a very horseradish-y taste and smell

Interesting question and even if I am just speculating, there is a probable chemical explanation to this.

If you are used to handling horseradish, you may have noticed that it must be cut or grated to produce the typical smell. What actually happens is that myrosinase and singrin from the broken cells react and produce allyl isothiocyanate, the compound causing the typical pungent smell of horseradish, wasabi and mustard. If left open in air, the grated horse radish looses its smell, since allyl isothiocyanate is rapidly oxidized (it reacts with the oxygen in the air) and form other compounds.

Now, kale contains both myrosinase and singrin as well, but not nearly as much as the horseradish root. Probably, the content is so low, that the produced allyl isothiocyanate is oxidized so fast, that the pungent smell is not noticed. If you however throw in some lemon juice, the contained ascorbic acid acts as a strong anti oxidant, preventing the reduction of the allyl isothiocyanate and potentially keeping a noticeable smell.

If I am right, sprinkling lemon juice over freshly cut broccoli should have about the same effect. I don't have any at hand, but it would be interesting to know if someone tries.