I can't find a reference for this right now, but here's how I heard it explained by some WAPF/PPNF approved nutritionist or other:
Grass-fed lard or beef tallow has high levels of vitamin E and other antioxidants. Over time, these get used up in protecting the fat from oxidation, so it becomes somewhat less nutritious, even though it's still perfectly usable. Once the antioxidants are all used up, it will become rancid. This will all happen more quickly at warmer temperatures, if the containers aren't sealed, etc.
The fat from grain-fed animals is lower in antioxidants, so it doesn't last very long, especially at room temperature. That's why the manufacturers started hydrogenating it, or adding BHA/BHT.
I don't know what the difference is with the plant fats. Are they more completely saturated, maybe?