Doesn't hurt to be a little bit paranoid, especially with these babies in particular. And by the way congrats on your babies. I absolutely adore barbu de watermaels.
Sometimes it's caused by a defective sperm or the condition of it's environment to which the cells weren't paired correctly. I'm leaning more towards the sperm.
When a chick breaks from the usual behavior it's a early sign of poor brain development. If you kept it, it would be a special needs bird and not breedable.