I think part of it is also the change of pace.

I mean, I often get some good thinking in while driving about on autopilot, chauffeuring the kids about, but a few weeks ago, my car was in the shop for a few days so I was driving the wife's and trying to make sure everyone still got where they needed to be and got picked up in a timely fashion.  It wasn't my car, and for some reason, I felt obliged not to switch my wife's music for something more my taste, and mostly that was just bugging me.

Then one evening, as I was carting kids to a session at one of the more distant libraries in our district so we'd all be out of the wife's hair while she taught piano lessons at the house, and some combo of the odd car, the odd music and the odd drive gave me a sudden and overwhelming epiphany.  I almost couldn't get to the library quick enough.  Buckled the kids into their favorite activities, planted myself at a table guarding the entrance to the kids area and whipped out my trusty old AS3K.

I wrote nearly 3k words worth of notes, mainly as regarding said epiphany which looks to cover most of the dreaded middle of my WIP, but once the floodgates were opened, I got notes on a couple of other projects that have been on/off over the past couple years and even a brand new thought on an old concept that as far as I can tell, had never existed anywhere but in my head for nearly two decades.

All that in 40 minutes ... powerful stuff.  Sure its wonderful when it all comes together like that, but I don't think I could survive if every day of writing was like that ;-p