A fragment 0.5 meter in size would not be falling at over bullet speed, and would not land explosively; instead it would be at terminal velocity in air, which is pretty low. Subsonic probably. It was ejected from the explosion 30-50 km up, then it fell, slowing down to terminal velocity as it did so. The impact with the ice could then have shattered it. Recall my earlier post about threshold size; much above that size meteors do not lose much speed due to atmosphere hence land explosively; if much below it they lose essentially all speed and end up at terminal velocity in air, which is low (and size dependent) and if small the meteorite will be pretty undamaged.