I'm pretty sure it has to do with the amp draw. It only happens in my electric car, my 8th scale buggy actually got pretty sluggish towards the end of a 15 minute main recently. I had topped it off before the race, and thought it had enough in it. When I later tried to cycle it, there were 0 mah left in it! (@ 1.05v per cell cutoff) It's a 1400 mah 5 cell, and should have made it through the night.
I figure having a low turn mod in my touring car, with it's large amp draw, might kill the battery's voltage enough to make the DSM kick. That's the only car I've had trouble with DSM in. Now that I have plenty of battery for the race, I never have that issue.