The MOP warrior challenge set use to set you shoulder on fire when using a skill or a buff if i recall...
Since Legion or maybe WOD, it only procs on emote ?
There is also the sort of shielded animation that is used to proc at the same time that seems to have disapearred...
I believed it used to proc when you triggered enrage as fury warrior
It still procs when activating certain abilities (Battle Cry, Enraged Regeneration etc.) or during sprinting. It will also proc when using Battle Shout in BfA.