The most likely explanation is that the Ukro sams shot the missile down and instead of hitting it's target, it fell on the nearby hospital.

However Russian sources are reporting that the missile did indeed hit it's target, which was next door. And the hospital was damaged from the secondary explosion from all the bombs and arms that were stored next door. This would make more sense and the ukros have repeatedly stored ammunition in densely urban populated areas in hopes that russian's wouldn't strike them to avoid bad optics.