Fairly common actually.  Most hens will eat them if they break, so fragile or soft shells are a major trigger for it, and then they just have a go for them after the first time or two.  Boredom is also something that aggravates things, but yours sound like they have lots of space to roam and bugs to hunt etc.  No matter how it started though, sometimes they'll kind of forget about it, and there are a few things will help. 

Gathering eggs more often for one, though if the hen is one of those who turns and attacks her own egg quickly, it's hard to do.  If she's one that goes snooping after other hens' eggs, it can help though.

Making the place where they lay as dark as possible, often by hanging a 'curtain' over the front of a nest box if you use traditional nest box set up.  Farmers used to just hang a bit of burlap sacking over it, but I've seen on the 'interwebs' people who actually sew frilly curtains for their hens… errrr, last time I did it I use some old denim from a pair of cut-offs and a few nails.  After a while they seem to get out of the habit and it gets forgotten… they don't call them bird brains for nothing!!!

Last but not least, some hens never quit.  They leave the flock.  How you make them leave is up to you.  I prefer a nice pot of soup myself.