You're correct that each fragment will have its own independent orbit, based on the delta-v that it gained from the explosion. But they won't circularize; each

You're correct that each fragment will have its own independent orbit, based on the delta-v that it gained from the explosion. But they won't circularize; each of those new orbits will intersect the original orbit at the point where it exploded. The orbital periods of the fragments will also not match the original, meaning that the relative intersection will shift along the original orbit every time. So an explosion in geostationary orbit would be a Really Big Deal and could potentially threaten the whole geostationary fleet.