It can happen when they first come into lay and also if they are young. Make sure they are getting plenty grass and we give ours flaked maize (looks like cornflakes but just maize) or mixed grain, we give ours an iceberg lettuce (Tesco 49p) to share once a week (we have 6 geese) which they love, especially when there is little in the way of spring grass and a bit of brown bread now and then is good to feed them up before they start sitting. I wouldn't worry but worth keeping an eye on as Louise says.