Problems with reverse

I was parked pointing uphill on a very steep grade earlier today. There was a car in front of me and an empty driveway behind. I put my '06 package 3 into reverse (the "R" was blinking and the picture from the back-up camera was showing. No, it wasn't beeping because I disabled that function the day after I bought the car in May 2006. And that damned beeping was how I found PriusChat, and we all lived happily ever after...). Are we all on the same page? Car's back end is downhill, car is in reverse, and parking brake is off, yes? I take my foot off the brake and the car moves FORWARD (i.e., UPHILL).

I ended up having to resort to my old manual-clutch-in-San-Francisco trick, kinda--since I WANTED to go backwards--of using one foot on the brake and one on the accelerator at the same time. TO ENGAGE REVERSE. WITH GRAVITY ON MY SIDE.

I've never experienced anything like this with this (or, for that matter, ANY) car and am purely stumped. The only thing officially "wrong" with the car right now (that I know about, I mean) is that the maintenance light is lit, but I think that's because I'm due for an oil change. Well, I KNOW I'm due for an oil change, and I don't know of any other reason that light would be lit now.

The car had about 3/4 of a tank of gas, and I'd been driving through some intermittent light rain showers for about 25 minutes, and it had been sitting parked for maybe 5 minutes when I started it up again. It was around 70F/21C.

Any ideas why my Pri is pranking me?