Was it actually a slide or just the feeling of the back end twitching?
It may be worth checking the rear subframe bushes.
When these wear they cause the car to wander as the subframe moves.
I'm thinking that this could be made worse in the rain at speed if you get a build up of water infront of a rear tyre that stresses the bushes momentarily.

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'02 X5 3.0d, Auto, Topaz Blue