What is WebKitDropDestination?
DropDestination is Apple's developer terminology for the destination of a Drag action, e.g. when you drag something with the mouse from one app to another. 

WebKit is Safari's HTML rendering engine. WebKitDropDestination appears in the WebKit source code and is part of the drag and drop code. The folder is intended to be temporary.

There doesn't seem to be any formal documentation of the term WebKitDropDestination or other references online.

If you have indeed dragged a bunch of your videos onto Safari or an application that uses web views, to upload them to a site, then this is the likely explanation.

If you haven't done anything of the kind, then it may serve some other use. Folders of this kind usually get deleted on a restart.