People hate change. Worse, people hate surprising change.
Mike is mostly mocking the people upset with this on Twitter, but if you enjoy a show made up of two people, people get annoyed when one party leaves with no explanation.
Lots of people say "they don't OWE us an explanation". I disagree. People invested in 5by5 through buying shirts, buying the app, listening to ads and using their promo codes, and the time they spent listening and interacting with the network. And when you sign off a show saying another one is coming, and then switch networks and drop 1/2 of the show with no explanation, that's rude.
The way this went down, there's two possibilities:
1. Dan had no idea, which is why he got the shirts printed and TTS was still on 5by5's recording schedule as of Gruber's announcement it moved to Mule.
2. Dan was in on it, but had agreed with Gruber and Mule to keep it secret, for whatever reason.