Hi Jeff Mills & Co.
That's odd! Has this issue only cropped up with that second batch of five that you tried to combine, or has it occurred with other groups of files? CombinePDF should honor the page orientations.
Are all the PDFs you're trying to combine in landscape orientation? (CombinePDF should honor page orientation even when you combine files with different page orientations, however.) What happens if you combine fewer of those five files? I'm wondering if there may be one or more files in the lot that is damaged, and causing the issue you're seeing.
I look forward to hearing back from you.
Best,
Sara