Brakes pulling the car in one direction usually indicates seized or stuck calipers. However this only occurred intermittently and in heavy rain creates more of a mystery. I'd have the dealer re-check the calipers to ensure they are functioning correctly.
2013 X1 28i Sport - AW/Coral Red/Ultimate/Cold/Lighting (Delivered on 12/8/12)
2011 X5 35i Sport Activity - AW/Blk/Aluminum (GONE)
2005 X3 3.0 (GONE)
2006 Subaru Legacy GT (GONE)
2001 Honda Prelude modded (GONE)