I just made a fruit smoothie with Kale, banana, Apple, blueberries, vegan protein powder, chia and flax. It was like drinking a smoothie with horse radish. The blueberries were the only difference with my favorite smoothie. Did the blueberries cause this drink to be spicy hot?
There's a chance the extra sugar from the blueberries has helped bring the peppery taste from the kale out a bit more. Kale is known for its light peppery flavor much like watercress but not any where near the heat of horseradish. The addition of sugar, much like salt is great for intensifying flavors.
Rather than try and describe the taste of kale myself, I'll use the words of someone else instead.
The flavor is peppery and assertive, and can have a pleasantly bitter edge.
Source: A Sweet and Savory Life <- A good read if you are more interested in kale.
So, in reply to your question. Yes, Kale can taste hot like Horseradish.
As a member of the Brassica family kale contains at least some amount of glucosinolate, which is exactly the stuff that makes horseradish hot. It is present in other Brassica family members as well such as white cabbage or the like. Some varieties of kale might have more of it than others. The thing with glucosinolate is that it looses much of its heat during the cooking process, which is why cooked kale mostly doesn't taste hot. I assume you used raw kale in the smoothie, which means the glucosinolates were still intact. So it is entirely possible that this time you just picked a kale with a higher concentration of glucosinolate than usual.