Israel claims an Islamic Jihad rocket misfired and hit a hidden ammo dump in or near the hospital.
Update:
IDF: An analysis of IDF operational systems indicates that a barrage of rockets was fired by terrorists in Gaza, passing in close proximity to the Al Ahli hospital in Gaza at the time it was hit.
Intelligence from multiple sources we have in our hands indicates that Islamic Jihad is responsible for the failed rocket launch which hit the hospital in Gaza.