It was literally an attack on a civilian center of a civilian city while ceasefire was being floated as a possibility.

That, and Russia used Belgorod as a launching point for several Iskanders missiles against Ukraine in the large strike. So a lot of return fire went to Belgorod to destroy the launchers. Missile off course, intercepted, Ewar jammer, so many reasons a single munition can end up in a population center. It's not as if a grad was used to carpet Belgorod.