Stuff like that usually happens as a combination of one or more of the following (this is by no means an exhaustive list):

• whale self-upvotes near the 6.5 day deadline
• lots of money used to purchase upvotes from bidbots
• spammer with botnet mass upvotes target post near 6.5 day deadline
• misfired voting trail (so one must be careful when utilizing Steemvoter, Steemauto, or other similar services)
• other forms of reward pool rape
• lack of a mechanism in the STEEM blockchain to recognize and automatically strip bad actors of their stakes

You get the idea. What @mattclarke described is the "polite summary" version. The fully detailed version would be long enough to fill up a book, whose content would consist largely of anecdotes of multiple Steemian psychopaths and thousands upon thousands of expletives.