This is known as a widowed group footer or orphan group footer. You can prevent this happen by following steps below :
1. Insert a section below the Group Footer to
create Group Footer 1a and Group Footer 1b.
2. Copy all the fields from the Details section into
Group Footer 1a. Group Footer 1a and the Details
section must appear in the same format.
3. Copy the original Group Footer into Group
Footer 1b.
4. From the 'Format' menu, click 'Section'.
5. From the 'Section Expert' dialog box, select
'Group Footer' from the list of sections and select
the 'Keep Together' check box.
6. Select 'Details' from the list of sections and
click the 'X + 2' button beside the 'Suppress'
check box. Keep this check box unselected.
7. In the 'Format Formula Editor' dialog box, type
the following conditional suppression formula:
//This formula conditionally suppresses
//the last record of the Details section.
//Replace {database.field} with the field
//by which you are grouping.
WhilePrintingRecords;
Onlastrecord or {database.field} <>
Next({database.field})
Upon completing these steps, the Details section
will display all the group's records except for the
last record. Group Footer 1a will always display
the last record. If the group extends to the next
page, Group Footer 1a and Group Footer 1b will
always print together.
Therefore, this solution will prevent the
appearance of a widowed group footer.
NB. this procedure is quoted from technicalsupport of businessobjects.com, not from my own.