You guys had some great guesses! You really thought of a lot of different ways those shells could have gotten where they are!
The shells were likely piled there because of a bird that specializes on molluscs. Birds like to find a really good place to eat their food. You will often see this with other birds of prey like hawks. A lot of hawks find a perch that they have found to be good for sitting on after they catch their prey and return to that perch every time they catch their prey.
Back to the shells- notice they are all cracked open and there is no animal inside! This means something must have cracked it open to eat it.
We didn't see which species was cracking these shells, but I did some research and here's one possibility:
The black oystercatcher found along the Pacific coast from Alaska to northern California. They use their long strong bills to dislodge food and pry shells open.