I'd bet that's the core of it. Gruber probably thought he deserved more money, and Mule was willing to give it to him. The 5by5 hosts bring the lion's share of the value to the network, and I can't blame them if they want to get paid as such. It isn't screwing 5by5, per se, but perhaps a little scummy (as Gruber might write)?
UPDATE: Other explanations I'd also bet on (might as well hedge, right?):
(1) Gruber wanted to be with his friends on the Mule network
(2) 5by5 was getting a little crowded, and Gruber wanted the Talk Show to stick out a bit more.
They get paid. Dan talks about it occasionally, around talking about FreshBooks, I think.
No idea how much, but I think it was David Smith of Developing Perspective that said a week of DF sponsorship was $7K and 5by5 $5K? Sure at least the latter must depend on the show. Plus I'm probably mixing things up. Episode 42?
They do. In an episode of the After Dark (called Pamphlet) Dan talks a lot about 5by5, the equipment he uses and various other things, including addressing questions about whether hosts get paid. He says, in short, that if you hear a sponsor on the show then the host is getting paid.
Dan is a professional and has my respect, where as Monteiro makes cheap vulgar jokes. I know which attitude I prefer to support.