Rashi as an explainer of ta'amei hamitzvos

Rashi (14:4) explains that the taharah process of the metzorah uses birds because birds chirp, symbolic of the chatter of gossip that causes tzara’as; it uses cedar wood because the metzorah was haughty and thought of himself like a high and mighty cedar; it uses an eizov because it is a low plant and shni tol’as because worms also signify smallness, which teaches the metzorah to have humility.

Is there some difficulty in pshat that drives Rashi to give a reason why each of these items was used? I couldn’t think of any. Compare with Rashi Bamidbar 19:22 where he explains why the same eitz erez, eizov, and shni tola’as objects are used in connection with parah adumah, but he does so only when he quotes the midrash aggadah of R’ Moshe haDrashan, not when he is simply explaining pshat in the pesukim. It seems that Rashi in our parsha is stepping out of  character from being simply a commentary on pshat (by which I mean syntax or language) and offering us ta’amei hamitzvos, yet if that is Rashi’s goal, why does he focus only on the objects and not on anything else in the process, e.g. why place things on the ear lobe, the thumb, and the big toe?

There are other places where Rashi seems to offer explanations that sound like ta’amei hamitzvos, e.g. Bamidbar 8:7 from yesterday’s post. Does anyone have a rule that works to explain where and why Rashi does this?