I don't think it was rigged. The Selena fan base was constantly on my timeline voting for her & retweeting each other. There are definitely more hard-core Selena/Demi stan twitters (considering most of their fanbase is younger) than there are for Miley.
I also wonder if they don't count votes from the artist themselves. A lot of Miley's votes came showering in after she tweeted about it. Selena had been leading the voting until Miley started getting fans to retweet her so maybe they threw those votes out? Anyways, when you compare the hashtags on most sites, It looks like Selena did truly win.