UFC 136: Why did Melvin Guillard leave Jackson's MMA to go train with Rashad Evans?

Because Melvin Guillard's coaches work for him, not the other way around.
That's according to the surging UFC lightweight "Assassin," who told Sherdog.com that he needs to get the best possible training for his upcoming fight against Joe Lauzon at UFC 136 on Oct. 8 in Houston.
Even if it means temporarily shacking up with "Suga," who may not be on the best terms with the boys @JACKSONS  Albuquerque:

"I know a lot of people look at it like, ‘How are you at Jackson’s and you train with Rashad?’ Well, Rashad’s been like a brother to me since Season 2. Jon ... he’s my boy, he’s my teammate. I stay out of their beef, and I keep myself neutral to everything. For everybody that don’t know, that’s what it is. I still love Rashad like a brother, and I love Jon like a teammate. There are gonna be other times when I’m gonna go to Imperial, and I’m probably gonna train there again ... Until it presents a problem as far as my coaching goes, then I’ll deal with it when it comes. I’m my own boss; my coaches work for me, so it’s whatever I choose that I want to do to be successful in this game. I’m going to take those options."