@Riot: Luden's Echo spell proc: What causes it?

Earlier this evening I played a custom game as Brand, and I noticed that Luden's passive would proc during his passive DoT. Finding this interesting, thinking it might be a bug, I tried a few other champions with DoT effects with Luden's. Some DoT effects procced the extra damage, and some did not.

For example, and DoT spells would proc, while poison did not. I didn't try any other champs (Anivia, Malzahar, Cass, Fizz, Heimer, etc) but it made me curious as to which DoT spells are supposed to proc Luden's passive, and which are not, and why? Thanks for your time!

I don't think this belongs in "Report a Bug" as I think this reaction is intentional, but if it is actually a bug, then perhaps this can be redirected there. :)