we've had some of them in the past, in fact one of mine is still laying them come to think of it. I think its sometimes young birds just starting to lay or if they have had any big changes (move house etc) or even diet - not enough oyster shell grit. ours is young bird who's usual nest box has been taken over by 'moody broody' ( I have to keep turfing her out to the sin bin) and I think its caused a bit of upset to this particular hen.