I'm always hesitant to embrace the insanity defense early, but I think this might apply.
Do you know those little crazy thoughts people get and then dismiss? Like when you're standing next to a busy street and think "Yikes, I better not move any farther forward" and that weird impulse to step out happens? (Please don't tell me I'm nuts, LOL, I've heard other people talk about it. I think there's a name for those impulses) Anyway, a rational, sane person is able to say "WhereTF did that come from?" and consciously take a step back.
I think some people might have that part of their brain shut down, temporarily or not, and just let that impulse take over. She might have thought "Wow, I better be careful with her, that's a long way down" and instead sent her plunging to her death. Horrible and inexplicable, but I often find myself amazed at the human brain and all the little alarms and triggers and safety systems it employs. Maybe hers failed.
That sounds weird, but believe me, I'm not one to seek the insanity defense first and ask questions later. I had a hard time with Andrea Yates pleading insanity until I found out all the details of her life leading up to the drownings. In my gut, this case says "irrational, insane impulse" rather than revenge or some other "rational" motive.
