I get them fairly often from some of my older layers, especially the old commercial White Leghorns, it does seem to be age related in their case. Shell gland problems (age related?) and stress seem to be mentioned for all of them (corrugated, wrinkled, body checked etc).. stress in a chicken can be things we don't really think about also, waking up at night for whatever reason, running around more than usual, some of the articles mention things like if a pen of chickens is upset it will affect the eggs of hens in adjoining pens etc for birds of any age. These commercial type birds also were not bred to be kept for as long as most of us keep them (commercial flocks will cull them at 2.5 years or earlier usually). Don't really know of a good way to tell which hen is laying what unless you trap nest or separate them, some people do put dye etc on the vents which may work with a small flock.