The less superficial reason is that with the sudden increase in work, I just can't justify doing the work for the class. If I slack on work, that's one thing -- I'm just being a poor worker -- but if I'm reallocating cycles that should be applied to reaching a deadline to reach a deadline for another class, that seems bad.
Both of these reasons, however, are really more effect than cause, and I'm working on developing better mental awareness. I dropped the class as soon as I realized I was thinking about it. Really these are evidence that I don't care, and stubborn academic pride is not a good reason to counter my interests. I know that these are because I don't care, because as fws pointed out, I have no trouble making it to my 8:30am Saturday morning agility class (or, not no trouble, but I put in the effort and am energized by the prospect). And I've stayed completely on top of my work for Managing Organizations, which has a much more aggressive demand-curve (the midterm was last night). Because that doesn't seem like work, it is more pleasureable and engaging.
(There are also financial and advancement implications -- I can take two MBA minis in the next session, and although I don't pay for classes, I will pass the tuition-benefit tax-free segment and have to start paying taxes on additional graduate coursework)
This is a new development for me. One can only wonder what my degree would actually have been if I had known one is not supposed to pick the least intuitive and hardest subject they can find, and stick with it to prove you can do it.