One part of the specified procedure for opening the bridge hatch on a fleet boat was to close the lower conning tower hatch first. If there was any overpressure present, this limited it to the volume of the conning tower, instead of the entire boat. Not having seen the German design, I suppose it's possible there was a difference in the latching mechanism that would make it harder to open the hatch gradually, too.

There were, by the way, also British reports of commanders being blown out of the hatch.

J.T. McDaniel