I have encountered the same bug. I only discovered it because my C drive started to fill up.
I had recently imported videos from another folder on my hard drive, but Lightroom seemed to have Cached hundred of other videos that were not imported or even in that folder.
Purging the Video Cache inside Lightroom Preferences did not work, instead I had to delete the contents of the folder using Windows Explorer.
In the end I created a batch file to delete the files on startup until this is fixed. See comment 7 on this page:
More information here as to what is going on:
I recently deleted more than 40 Gb of data from the video cache files and I suppose that I have the exact same issue as I sometimes import photos from folders that contain videos.
Yet another Adobe problem.