I am not sure that all of the heavy Flak mounts are missing their shields, but most seem to be. These were not turrets per se, but shields bolted to hard points on the mounts. It is possible that some may have been loosened by the effect of hits on the shields themselves or hits nearby, or by being struck by falling wreckage as the ship capsized, and subsequently being torn off by hydrodynamic forces as the ship sank.
Their shoulders held the sky suspended;
They stood and Earth's foundations stay;
What God abandoned these defended;
And saved the sum of things for pay.