It's that weird zone where the story just takes over. I've had scenes where I wrote a whole argument between two characters and only later realized I'd given one of them a limp for three pages without planning it. Your brain goes on autopilot and the details just spill out. It's equal parts thrilling and a little scary, like you're not fully in control of the wheel anymore.
Know that feeling all too well, @olivers28... I once got so into a dialogue that I wrote three pages before I realized I'd been typing "teh" instead of "the" the whole time. My brain just checked out while my fingers kept going.