Take Amy Hoy for instance (at least some people here on hacker news will know who she is). She has more than 16,000 followers on Twitter right now. I have 79 followers. She is good at marketing and it must have taken time for her to build up all of those followers. If she were to put several products on Product Hunt and I were to put up competitive products to each of those and we both tweeted about our Product Hunt posting naturally she will get more upvotes. Some of these upvotes will likely come from the same people, some of her most fanatical loyal customers.
To the algorithm this might look like collusion/voting ring, but it is just some basic and completely ethical marketing.