When you think about it, FWD should really be more controllable than RWD.
With FWD cars, you are guaranteed that the direction of the force is in the same direction that you are steering. A RWD car can make no such guarantee...
In RWD cars, the force is always being applied directly forward. What your steering does is, if the front wheels have traction, then they will transfer the force to the direction you are steering. This is why RWD cars work at all. However, in adverse conditions, like icy weather, the front wheels may lose traction while the rear wheels still retain friction. This will result in the car moving forward regardless of how you steer. Not a good situation to be in...
Adding weight to the trunk will only aggrevate the situation because it will give the back wheels more traction without doing the same for the front wheels, making the situation of pushing the car forward without steering control more common.
It doesn't sound like this was
mathew330's problem but, then again, from his description, his incident should have been impossible. I mean, really, what can cause a car moving straight down a straight road and then skid to the side for no reason?
If I were to grasp at straws, I could hypothesize that both front wheels and the rear left wheel lost traction while the rear right wheel turned the car counter-clockwise around its centre of gravity. However, this is highly unlikely in many ways, including the car turning as fast as the original post suggested.
The only thing I can say is that even 25 mph (which is more than 40 kph) might have been too fast for iced over streets. I recently drove around town ofter a snowfall and discovered some slippery streets on my way to drop off the rental car. I was doing a right turn in a right turn lane from one major street to another and felt the ABS kick in at only 30 kph (18 mph). Luckily for me, there wasn't much traffic and I could simply do my turn but it scared me. Now, I've only had my (temporary) license for two months now and I wanted to understand how to drive in adverse weather. So, I circled the block in order to do the turn again, this time without enabling the ABS. Luckily (depending on your point of view), on the other side of the block, from one side street to another, was another slippery intersection. I suspect this one might have had ice under the snow. Anyway, I also slid at this intersection while not even doing 20 kph (12 mph). In fact, I slid so much that I decided to abort the turn and continued down the side road to another less slippery intersection. Going back to the right turn at the first intersection, I saw what looked like only snow and slush but felt the ABS come on while doing only 10 kph (6 mph). Sad to say, I was getting tired of the exercise and was worried about delivering the rental car on time so I just continued on my way.
The point of this story is that, in adverse conditions, you may have to bite the bullet and go even more slowly than you're comfortable with. I know that we're all used to going at "reasonable" speeds in good weather but when that water falls on freezing ground, the definition of "reasonable speeds" changes quite dramatically and there's nothing you can do but avoid snowy, icy roads and slow the hell down!