Long story short, Anime Matsuri is run by a man who is a known sexual harasser and thief. He has hit on underage girls, stolen equipment/money, and never paid some of their guests (including Miyavi).
Some more info if you want to read up on it:
An informative post by "Boycott Anime Matsuri" on Facebook
A post by a well-known j-fashion blogger about her experience at Anime Matsuri, as well as all of the lawsuits against the con