So, the grass is always greener on the other side of the fence. Dream Theater may have been Mike's baby, but it was also his job. Any job, including being in a rock and roll band, is still a job, and there is a degree of "obligation resent" even if it's subconscious.
Transatlantic, Hail, Avenged Sevenfold, all those other side projects he's done in the past years... those were all things he wanted to do. For the music, for the fun of it. Going back to "work" (DT) suddenly seemed like a drag, so he asked the guys if maybe they could postpone going back into the studio... and the rest is history.