The thing is, there is a red circle for that, he turns to launch the easily avoided rocket. Tanks should understand how that mechanic works, and DPS/heals should _never ever ever EVER_ be hit by it.
Not talking about the red circle. I think sometimes he simply shoots at another guy just to keep your healers from being too bored. The bonus boss on Cademimu randomly shocks someone that isn't the tank, again to keep healer from being too bored. It's a pretty common mechanism and sometimes people freak out to see the boss turn around to another guy for no apparent reason, because they're supposed to do that. These attacks are just meant to keep your healers on their toes.