Here is our articled on shell aggression.

http://crabstreetjournal.org/blog/2012/09/27/identifying-and-addressing-aggression-in-hermit-crabs/

During mating there will be some knocking of shells and rocking… I don’t remember if the crabs exhibit chirping at this time however.

I had this happen when I brought a new crab home.  I stopped it by giving the aggressor and the ‘victim’ a quick dunk in my water pool. It seemed like the aggressor was able to tell the other crab was new.

You may want to search for info on pre mating behavior also.

Hermit crabs in captivity are  denied food and water until there are enough of them to fill a huge crate to ship to the states. They arrive at petstores hungry and dehydrated. Most petstores offer a small bit of water and hermit crab pellets.  So if the hermit crabs live to come home with someone it’s not unusual to see them gorging or protecting food.  Keep offering lots of different foods so everyone is eating well and Onion will settle down in time. We have some articles on all the different foods they can eat.

A bigger tank… welcome to hermit crab addiction LOL

Seems like you need either another access route to the food or two food dishes in you tank if this is going on.

I sprinkle things to eat around too–maple leaves, crushed shell, cholla wood, barnacles, oak bark, coral. They eat the plastic plants and the clay pots too. They visit the food dish in order of dominance at mealtime, and the ones waiting tend to chew on the other stuff while waiting.
I never resolved this issue with my two crabs Baby and Sweet Pea though, who after 6 years have become jumbos.(I have 3 quarter-sized crabs too).Sweet Pea has been a bully from the start.I have always sensed that Sweet Pea at some time was not well treated and survived by intimidating other crabs.

Do not dig the crab up, it may come up by itself and look for a shell. Try to make sure meanie can’t get to her when she comes up.
Coax the crab towards her shell, if you need to, you can put her into a cup with some distilled water and her shell in there, she may change into it. There is a link on this website explaining exactly how to do it. A little bit of water so the crabs abdomen doesn’t dry out.

I know what you mean Beth, went from a 20gal to a 30 or 40gal and now want something about 72″lx12″wx18″h so I can have room for a good sized ocean pool and have some heat and humidity variation end to end.