Crusader AA MK 2

[Astronomican]

The photo above showing the Oerlikon-armoured Crusader AA has a bar directly in front of the rear hatch - this bar is linked to the guns. Although the guns can be fired with the rear hatch close, to get the best use of the guns that rear hatch was opened so the gunner could use the guns more effectively. By opening the rear hatch you made the turret vulnerable to splinters and grenades - hence Open-topped.

Also, many sources state that with Allied air superiority most of the AA tanks were withdrawn and the men assigned elsewhere. The only exception I know of is the Polish AA tanks, which were retained and used very effectively in the anti-infantry role.