My hunch is--just a prank. My concern is--my hunch is wrong and it's a dry run to test how we'll respond.
I'd classify it as an act of terror, since it obviously had political connotations, the grenades have a representative value of being made for killing people, and even though it was a tiny little explosion compared to what we're hearing about the terrorists using, the fear this could have generated would've been disastrous if it had actually managed to kill someone, however accidentally.