There are many possibilities...
Can be as simple as a glitch or as complicated as HPFP.
I got this massage 3 years ago (on my 650i) and it ended up to the replacement of injectors. There are many threads in M5post forum and most likely people who modify their car, get this massage more than others...
this is one of them:
http://f10.m5post.com/forums/showthread.php?t=1075579