Peter,
In this particular situation, this is what happened:
A corner was approached, assuming that (as he was on the roads) he was throttle neutral - he was neither accelerating nor decelerating. As the OP doesnt state it, I am going to assume that it was a constant radius turn.
As he executed the turn he lost velocity, transferring weight to the front wheels. This, combined with the lower traction offered by the wet road the front wheels started to slip. This is understeer. His reaction to that was "woah <lift off the throttle>".
This didnt help the situation as it transfers even more weight to the front of the car. There is now two things going on: 1) The front wheels have no grip because they have been pushed beyond the limit of friction and 2) The rear wheels have lessened traction because there isnt as much weight on them (its all over the front wheels) and so they now break traction. The back end of the car still has forward momentum but thanks to what is going on at the front, the momentum has now carried the rear end sideways. This is called oversteer.
Most cars are set up to understeer first (and usually shockingly early) as it is "safer" than understeer. Unfortunately if you meet understeer and you dont really know what you are doing you either hit the outside of the bend, hit oncoming traffic or get massive oversteer. Unless you are trying to provoke it, if you get oversteer in this fashion and havnt learnt how to deal with it you either get lucky or you spin out. Thats what happened to the OP.
In a RWD car, its a slightly different kettle of fish though!