Why are video and audio files re-imported sometimes when a project is opened?

The subject title pretty much is the question.  I've been working on a project with six shows inside now for a week or two.  Today when I opened it, all the audio and video files were re-imported before the project would open.  Why?  This is not happening most of the time, and can be a lengthy process if your files are large.  It makes no sense.  For the record, the last thing I do before closing the program, every time, is to do a save on the project.  Thanks in advance for any info.

    Duane