When a DCC locomotive does this, it is often caused by a dual mode decoder going into DC mode. The decoder thinks it is running on track controlled by an analog power supply, and thus will respond to changes in track voltage. DCC, however, uses a constant voltage on the tracks, at the upper end of what an analog power pack can deliver. Thus the locomotive takes off at full speed.
The good news is that this feature, called analog conversion, can be disabled by reprogramming cv29. The bad news is that cv29 also controls a lot of other things as well, such as normal direction of travel. Figuring out what value to program can be complicated.
There is a conversion table on the digitrax website that lets you enter the features you want, then calculates the value to program into cv29. Hope this helps.....
http://www.digitrax.com/support/cv/calculators/