Check out these two threads:
Brownout behavior - alternative design goals
How do you design a robot that doesn't brownout?
The issue isn't strictly limited to 6 CIM drivetrains, but to any system which draws enough current from the battery to bring the battery supply voltage (momentarily) below 6.3V. This is most likely to happen in a pushing match, and many (most?) 6 CIM drive trains designs are intended for pushing matches.
Edit: Another common cause for the brownout is running the robot for far more than three minutes without changing the battery. We haven't had this happen to us during competition, but it has happened several times during drive practice and at demos.