Mussel-ing in.

I don't know why, but the re-claimed front garden is rife with mussel shells.  I certainly didn't put them there.  Sure, we live in a coastal city, but we are not actually close to the shore.  You'd have to be a really persistent seagull to scoop a mussel out of a tidepool and then fly it all the way over to our house to drop.  Plus, now the front is soft dirt, so the shells aren't breaking in it.  

I feel I would have noticed them last year when I was grubbing about in that dirt, had they already been there.  I know that the freeze/melt cycle pushes all sorts of bigger things up out of the earth, but why all these shells?  Why now?  

I am flummoxed.