Whether a report prints in portrait or landscape is determined by a number of factors:
1) Number of columns,
2) Width of each column which is determined by the size of the longest text in that column,
3) The body font used,
4) margins,
5) printer and printer driver used, and
6) the Report Layout settings for orientation and smart print setting to reduce columns width, switch to landscape, reduce font when the report will not fit on one page.In the case of the Detail Trial Balance, the width of payee names and size of amounts determine whether it will be able to print in portrait or landscape mode. By displaying the report in print preview you will know exactly how it will look before it is printed.
You can change Report Layout setting to not switch to landscape. In FN version 8.0, this setting can be saved differently for each report.
Also you can reduce the width of columns in order for it to fix in portrait. In FN version 8.0, the changes to the column width will automatically be remembered for each report.
The body font can be changed to a smaller font. In FN 8.0, the body font is automatically remembered for each report.
Hope this helps to understand when a report is switched to landscape and how to control how the report is changed to fit on a single page.