Why does my cauliflower puree taste mustardy after two days? I recently made cauliflower puree following this recipe.

I put florets of cauliflower in a microwave oven safe pot, together with some butter and some milk, and cooked it till more or less tender. Then I added salt, pepper, a bit of nutmeg and some cheese (I used cheddar and emmental).

At that point in time it was fine, like I had it before.
About two days later, I wanted to eat the leftover puree (which I stored in the refrigerator), but its flavor had changed. There was now a distinct sharp flavor present, which reminded me of horseradish or perhaps mustard.

Does anyone know why and/or how the flavor changed?
The flavor / odor notes you're experiencing are probably sulfuric compounds. Cruciferous vegetables like broccoli and cauliflower contain sulfiric compounds that are released on cooking. That's why the stir-fried crisp-tender broccoli tastes so different from the limp overcooked olive green florets. Similar to how spicy salsa fresca grows ever spicier after a couple days in the fridge, your puree is slowly continuing that "cooking" process. 

It's still okay to eat, but you'll be less aware of the flavor change if you use it in a more highly spiced dish once it's a couple days old.