You would need to know when you have photos in portrait if you wish to maintain that orientation afterward.
Most bulk image programs, whether resizing or doing anything else, need to have a consistent "long side" and "short side" and that's generally a landscape orientation as they work through the images. Bulk Image Manager intelligently detects if it's been handed a picture in portrait orientation and rotates it to landscape before applying the resizing algorithm so that no picture content is lost (except what always gets lost from any resizing, downward, anyway).
I wish it were smart enough to remember that it had done so, and apply the counterrotation to the end result, but it isn't. If you find a bulk resizer that is, please let me know. I haven't.
Brian - Windows 10 Home, 64-Bit, Version 1703, Build 15063 (dot level on request - it changes too often to keep in signature)