Why are some travel mats slippery when new?

This comes up in nearly every r/yoga thread we read on Gaiam and other PVC/TPE mats: a thin manufacturing film causes initial slipping. Reviewers recommend wiping the surface down with a damp cloth and a small amount of mild soap, then breaking the mat in over a few sessions. Natural rubber mats like the Manduka eKO line don't typically have this issue.