Well...um, the sword is cursed. So he can't control when he goes berserk or not, unlike using his normal ability.
I've found that when Minsc goes berserk even using his normal abilities, he sometimes goes haywire and attacks the party when there are no more monsters to fight and the berserk state hasn't ended. I don't know if this is a factor of his wisdom (which is low) or not, but I'm always careful to make sure he's in the thick of battle and away from the others when I have him go ape sh**.
Cheers,
-Saz