Dunno for certain, but I'd say some kind of artillery round that spread antipersonnel submunitions.
Those rounds are (or were in my days) known as FASCAM. Artillery cluster munitions. Most likely delivered by MLRS units. They can cover huge areas with cluster munitions that act as conventional mines, or the whole bunch can be command or time detonated. 6 rounds can cover a grid square with submunitions in just seconds. Sucks to be you in the middle of that!