HCA: Hermit Crab Association

I had 4 pp. all of them medium sized but 1. biggest guy dug down 2 months ago, Then second to biggest dug down a week later. Then i bought two new ones maybe three weeks ago. they immediately dug down followed by the third of my original 4. Now there's 1 little crab who doesn't do anything and my tank has become very boring. What gives? lol What are they plotting down there....I know they are probably molting but it seems like they all decided to molt at the same time. is this common?

You are going to look in one day and there will be 2 dozen shells and miniature electronics. They look so innocent but you know their new stuff didn't just fall off a truck lol