Hanging in the balance?

Last week, after finishing field work in the rocky intertidal zone, Eric looked up to see something odd.  There was something hanging vertically from the corner of a boulder:

 

Huh.  We both wondered what it was, so we moved closer for a better view:

 

Weird!  Now we could see that it was a stack of small limpets hanging down below the rock.  Neither of us could recall seeing something like that before. 

So I had lots of questions.  How did they end up in that position?  Did they just start "stacking down," with one limpet moving down and then another one moving down a little lower and so on?  Why would they do that (i.e., leave the solid rock and move into what seems like a tenuous position)?  When there is high density of limpets, and it's hot, do the limpets crowd into the shaded overhang, where some limpets are forced to crawl onto each other (below the tip of the rock)?

Is there only one limpet holding on at the top?  Can they crawl back up to the rock?  (Or is there a risk of upsetting the balance and dislodging the entire stack?)  When the incoming tide/waves splash the hanging limpets, how will they respond?

We'll never know the answers, but it was an intriguing way to end the morning.

Have you ever seen a limpet stack like this?

P.S.  We didn't have much time, so I'm not sure of the limpet species, but it appears to be Lottia digitalis or Lottia paradigitalis, or a combination of the two.