Based on the location and geometry of the fractures, I would presume the damage was cause by shovel diggers.

They were either abandoned due to the digger knowingly having cracked the clam and moving on... OR... the digger was unable to retrieve the clam due to a large incoming wave/surge causing the digger to abort and retreat upbeach.

Most of those clams are toast, but obviously some survive the injury and heal their shells. The big key to survival is how much internal damage is done to the clam.