I had this happen to me on two occasions. I freaked!
Oddly, it hasn't occurred now in over a year.
Even after performing the service procedure for shuddering under braking, the problem has resurfaced, albeit to a much lesser extent.
Can't wait to unload my Maxi & get my new 530i next year. Finally.. a REAL car with REAL rear-wheel Drive AND an interior not designed by Mattell.