Why do my imported media clips already have markers and why do they re-appear after I clear them?

So basically, I imported media files from a folder for my editing class and when I open them, there are already markers on the clip timelime in my source panel. How is this possible if the files were just mp4s? 

Also, I tried clearing the markers, saved the file, closed it, and reopened the same file again, only to find the same markers re-appearing as if they were never deleted.