WARNING: This page is about a different car, the 2013 BMW X5, 2012 BMW X5, and 2011 BMW X5. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: USB Overloading Voltage Graph
Courtesy of BMW OF NORTH AMERICA, INC.
The vehicle is not able to deliver the required level of current. The voltage (1) collapses and the current is thus limited.