I also had a similar problem. My housing on the size of the upper hose connector on the radiatior developed a 3 inch long rupture and we could not understand why. Anyway checked had the radiator replaced but could not bleed the system properly. Eventually checked the Thermostat and upon testing realised it was not opening. This in turn caused the build up of high pressure, forcing the radiator to blow.
Obviously this must be some form of design flaw in the Thermostat as the garage had encountered this as a frequent problem. Surprisingly enough no recall or information available from BMW!!!