I think I've found the problem. When I was in Afghanistan everyone was required to have their weapon with them at all times. That meant everywhere...work, the BX, the chow hall, the bazaar, even the communal crapper. It also made for a very safe environment. Everyone armed and virtually no crime.
Apparently two suicide bombers mixed in with the crowd leaving prayer services was the biggest cause of fatalities (about half). They had infiltrators observing and planning this for a long time.
These Corps bases are the Headquarters for the whole region of the Country. The one attacked here outside of Mazar-i-Sharif governs the North, with its large Uzbek and Tajik populations - which have been long time rivals of the Pashtun Taliban.