We have had several accidents over the years with "INERT" ordnance item. We used to send EOD Techs to the EOD School every few years to 'Recertify' all of the Inert training aids and there were always a few items that were found to still contain some energetic materials.
This was probably a case where the main charge was removed but the detonator was still live. Even the smallest items (about 1 oz. of H.E.) will kill or maim if detonated within a meter or so.