By chance?

[BrianSoddingBoru4]

The Sagan thing is very speculative, although the resemblance to a samurai mask is remarkable, the pattern reflects where muscles attach to the shell.

Of course it’s speculative, but it’s also reasonable - a random mutation reinforced via selection.  Whatever organisms prey on these crabs, humans are the only ones likely to be deterred by a chance resemblance to a human face.

Boru