It's also possible that you witnessed a "molting" event, as your observations are closely aligned with such an event.
Crab molting is nothing short of amazing. To grow larger the crab must back out of it's old shell to grow a new one. Not only do they back out of their shell, they also back out of everything, mouth parts, eye stalks, guts, and even manage to somehow back out of their gills! The resulting empty shell is called an exuvia that contains an empty shell of the entire crab - guts and all. Populations of crab tend to molt together. The females molt in the spring before the males - mating takes place shortly after. After such an event the beach can become littered with literally thousands of empty shells and body parts. If it is close to mating season you would most likely see a female molting event.
I found a couple of good pictures from the ODFW website....
- odfw comp..jpg (21.48 KiB) Viewed 1828 times
Here's the website if the picture doesn't come through well.
http://www.dfw.state.or.us/mrp/shellfis ... istory.asp