Snail colony -- more questions than answers

I took this pic while backpacking the west coast trail.   We tried to walk the beach, whenever the tides permitted.  One day, we discovered this colony of sea snails huddled together clinging to a rock that would soon be covered by the rising tide.  The snails are so small a penny would cover a few of them.  
What I wonder is why are they so closely packed together?  Are they drawn to each other?  Are they each vying for a prime patch of real estate that has the perfect combination of food and foothold?  Are they procreating?  Are they just huddling together for protection, with those on the outside most vulnerable?  How long do they stay clustered together like this?