Almost the same thing happened to me yesterday. Rear wirper went nuts soon as I started the car - went to turn off but the stalk was already in the off position. Put it in the first "on " position and after another sweep it stopped. Weird!
I also sometimes look in my rear view mirror to see the wiper stopped maybe 15° from horizontal, even if i havent touched the wiper stalk, but never see it get there
It makes me giggle every time.
Seem to remember reading a post which suggested problems like this were down to a faulty motor, but would also appreciate a little more information!