I see some merit here. However I echo EchoMirage in that it needs much more to be a full sub. I can deal with silly subs but this could be so much more.
Perhaps as Rahmir stated that it is an item of a Trickster god... perhaps a warrior/Palladin of much renown received a boon from his god in the form of a shield in which case, Loki (Enter appropriate annoying, devious and trickster type deity here.) put a hex or curse on it to taunt and annoy people who encounter said warrior/palladin.
Perhaps it will only goad those that it knows will give the wielder the most difficult of fights. Sure, it taunts and curses at a poor drunkard in a tavern and get him riled enough to step up to fight, but what the bearer of the shield doesn't realize is that the other six patrons he is drinking with find it also offensive and annoying.
Just some thoughts. I really like this however and could go to town myself on a history/ back story for this.