Watch that scene in motion. The pulses aren't slewing very rapidly and they clearly aren't tracking any moving targets. On the contrary, they seem to be shooting at something that is otherwise stationary with respect to the Kelvin and the very little movement we see is accounted for by Kelvin's evasive maneuvers.
Which means they're probably shooting at Narada's torpedo launchers or the tactical officers' best guess at where those torpedoes (or their targeting beams) are coming from.
Which is a smart move when it becomes evident that they are no good at anything else. Doesn't mean they would be any good as antiship weapons, either, though.
Well, Narada does completely stop firing at them about thirty seconds later. Which is good, because that's the same point at which Kelvin's weapons are completely out of ammo.