what causes this?

[mxpres]

one of my comets lays a very thin multi colored egg everyday,the shell is so thin sometimes I break it just picking it up,she is the only one that lays such a thin shelled egg,the others are firm

 

[Ridgerunner]

My guess is that it is genetic if she has always done this and your other hens are doing OK.

Soft shelled Eggs
http://www.thepoultrysite.com/publi...ndbook/16/thinshelled-eggs-and-shellless-eggs