It is typically commonly seen in metamorphs in which either the parents or the tadpoles have been fed an improper diet (the vast majority of SLS is nutritionally based) but there are many potential causes for it.

Ideally you shouldn't see it at all in a healthy clutch.

If you need to euthanize them then you can review the following article. Its a little out of date and should be updated but its still useful.
http://www.caudata.org/cc/articles/euthanasia.shtml

Ed