When you export a GEDCOM of someone's descendants and their spouses, if any of the descendants happen to have married siblings and you have their parents listed in the original database, those siblings will go to the GEDCOM file without their parents. They will be linked as a family with no parents. Now, if you are importing them into a database where they are listed along with their parents and you merge them, they will end up in two families, one with no parents. Deleting that family is much easier than unlinking them individually. A lot of times after a merge you can end up with two complete families except one will have only one parent. Deleting that partial family is much simpler than unlinking each individual.
DO A BACKUP BEFORE TRYING THIS!
If you want to get rid of that family with no parents, but keep the children in the database and linked to other parents etc. just highlight one of the children in family view and use the arrow at the right of the display until the children are listed with no parents.
Click on either "Click to add father" or "Click to add mother" and add anyone as a temporary parent, it is only for a few seconds.
Then Right click on this new parent and choose "Delete -> Family" Leave the default selected "Unlink family members" and click the delete button.
The children should remain as children of the listed parents, but the unidentified linkage will be gone.