What are my crabs doing?

[RenHoek104]

I have 2 crabs that appear to be bullying a third crab. It started with a chase between the vicitm and the smaller crab. Afterwards, big daddy crab showed up and appears to be holding the victim's shell.

Is this mating behaviour or am i going to lose a crab to crab on crab violence? I've attached a picture, but my Playbook's camera isn't that great so it might be hard to see what's going on.

Thanks
Ren