Did you author the discs yourself? Sometimes if you obtained these through some other means, previews and studio logos before the menu could have been ripped out, but then blank redirects were left in, rather than changing the disc authoring to skip them altogether, with a program like PgcEdit. So your DVD player sees the disc, tries to start playing a studio logo, sees a "skip to next video" marker, looks for the next video, tries to start playing a preview, sees that it's missing and theres just a command to skip to the next video... If a disc has a few previews before it that were ripped out, this can take a lot of seeking by your DVD player!