This occurs when families are on special payment plans. Either the family was on a school-use only plan or most commonly the payment plans of families had the "Extend Payment Plan" option applied on their account and turned it into a special payment plan. Designated late payment plans do not roll over year after year. This results in payment plans that are not recognized in the upcoming school year. So when the family is re-enrolled they are switched to either the default payment plan (if one is selected) or the longest payment plan available. 
Note: You are able to change payment plans while they are still in Pending Families.