I am not entirely sure about this, but i think that status effects like Sundered are counted as procs and due to that cant proc sets.

Been an issue for a while. Appears to be intentional:

https://forums.elderscrollsonline.com/en/discussion/617574/ravager-set-is-currently-bugged-and-not-activating-the-effect

https://forums.elderscrollsonline.com/en/discussion/626533/pts-update-37-bug-reports-for-combat-class-abilities

@Jierdanit is correct, status effects are procs for some unknown reason. Ravager used to be an AMAZING set in its original form. But it was nuked after the introduction of Relequen, which procced the original 5th piece: "When you deal melee damage, you have an 8% chance to increase your Weapon Damage by 645 for 10 seconds." Then came the change that sets cannot proc other sets, followed by procs can not proc other sets. And viola, neutered Ravager.