Perhaps they didn't trust that you actually owned the DVDs. As far as I know (though I am not a lawyer), it is legal to make a "backup" copy of content that you OWN (as long as it is for your personal backup use only).
But it is very very illegal to rip DVDs that are rented or otherwise do not belong to you.
If you didn't have the DVD discs in question with you in your luggage, the officers would have no way of knowing whether you actually owned the content or had stolen it by ripping rented/borrowed DVDs.
So, I guess the moral of the story is not to travel with ripped DVD content unless you can prove that you own the discs from which you ripped the content. A receipt should suffice, I would suspect, if you didn't want to carry around the discs.