'Cause users (who have just loast files) tell me they were dragging files from one place to another when they lost them.
I've also dropped files myself. A twitch of the finger at the wrong moment, and the file (or, god help you, the pile of 3 gazillion files that you selected) gets dropped in the wrong folder. I try to avoid drag and drop - I tend to select items and ctrl X them then got to the new location and ctrl V them.
Did you ever have to try and locate a pile of lost email messages that a user lost? Same problem. Drag and drop within the mail program, and dropped into the wrong place - bzzzp! Gone! Now try to collect those messages from wherever they got to, and sort them out of whatever was in that folder to begin with.
Drag and drop as a file management tool is a Bad Idea (tm.)