Cheating is a major concern in contests, both for the entrants who want a legitimate playing field and for the sponsors, who don't want negative publicity. An example of how a voting contest can turn bad for a sponsor is when Oprah was accused of rigging a contest when an entrant received hundreds of thousands of votes in an hour.
Sometimes, people do cheat on voting contests using bots or other methods. A sudden influx of votes could mean that an entrant has exploited a flaw in the code to cheat their numbers.
However, in today's day and age, there are some people who have a lot of pull online, and can rally their friends and acquaintances to legitimately score vast numbers of votes in a short amount of time.
For example, readers accused sponsor ThePamperedPup of rigging a contest. However, the company responded in the comments, and explained the influx of votes for one competitor:
[the contestant's] husband was an executive with The PGA of America. He had access to an email list that contained hundreds of thousands of addresses. I believe it was ten days prior to the conclusion of the contest that a mass email went out to all the addresses contained in that list. We fully investigated the tens of thousands of votes that came pouring in as a result, and determined that there was in fact no cheating that occured [sic].
If you're up against someone who has access to an email list of hundreds of thousands of names, or whose entry gets promoted on a local news station, or is able to get their entry to go viral for some other reason, a huge jump in votes can be completely legitimate.
And there's a third possibility as well - that there's a technical problem with the contest that's not your competitor's fault. Unfortunately, sometimes mistakes are made, and it's possible that something could be wonky with the contest's code, and it's completely accidental. So don't assume the sponsor or your competitor is cheating when there's a chance that no one's to blame.