Why orientation flips for some pdf (landscape to portraid) while openeing in AI?

I have found strange issue with Adobe Illustrator (CS5, WIN 7).

While opening some pdf which are landscape (originally) in AI, it automatically flips to portrait. It's not happening for all but for few.

We are writing script to process input pdf and convert to dxf in batch mode. Is it possible to identify and modify through script in illustrator?

Thanks for your time.

Regards,

Nathaniel