NHTSA's Monthly Defect Investigations Report calls the problem "Vehicle Rollaway Allegations," which are likely related to the design of the gear selector in the E65/E66. This generation of the 7 Series moved the traditional gearshift lever from the floor console to the steering column to make more room in the center console. In doing so, BMW abandoned the traditional P-R-N-D layout and adopted an electronic push-button selector for "Park." The AP says at least some of the problems may be related to owners not realizing their cars were in neutral instead of park.
Our own experience has shown that even in more recent BMW's with floor-console-mounted shifters, it can be difficult to tell whether the vehicle is in Park or not, and we find the BMW gear selector as a whole to be non-intuitive – we're frankly surprised that NHTSA has not received more than 50 complaints concerning the issue.