Doing it on purpose is Cheating - Disqualification but it's hard to prove. If you do it often enough, you might get flagged by the DCI and potentially get banned for sloppy play/cheating.
This goes for pretty much every infraction. And this is why they're all recorded and tracked by the dci. Realistically anyone could probably "accidentally" drop part of an opponent's deck while shuffling (or cast a spell for one less mana, or anything else) once and have minimal consequences because there won't be anything to suggest that it was intentional. But if someone is getting a warning for the same thing over and over, once or twice per event, but never more than a warning, this gets noticed. There have been dci suspensions for "accumulated warnings" before.