I think I know what you did, or at least I've had times where (especially when I'm tired and at the end of a ride, and early in my clipped in learning curve) I would be unclipped with one foot on the ground, off the saddle, but then move as if I think my other foot can just easily move to the ground and forget that I need to twist it. By the time I realize, my body weight is taking me over to the side that is still clipped in. Also, the first few weeks, what I kept doing was unclipping the right, off the saddle but still pretty upright over the top tube, then I'd look over my shoulder, which would throw off my body weight and down I'd go. It got quite comical actually. Now I've learned if I'm only unclipped on one side, I really move my body weight to that side, even so much as sort of leaning the bike towards that side.

Knock wood, I haven't fallen in a couple years.