Why do animals keep evolving into crabs?

An interesting and recurring trend observed in evolutionary biology is the seemingly inevitable occurrence of a crab-like anatomy.

This is actually an understood and fairly simple to explain phenomenon, resulting from the convenience of their anatomy, which is excellent both offensively and defensively.

A crab is a shrimp-like form in which the abdomen is tucked under the body, and in which the head and thorax are fused into a thick, heavily armored and calcified plate.

All of this makes the crab a difficult nut to crack for its predators.
Placement of the abdomen under this armor plating is design genius by evolution, because the abdomen is the most vulnerable region in case of a predator attack. By eliminating this weakness, the crabs rapidly rose to marine prominence.

The large claws allow them to crack open even mollusk shells. Prior to this, few predators were able to break into shelled organisms. Crabs evolved the morphological means to make many previously impregnable skeletons vulnerable. This gave them a significant weapon.