"Runaway" is generally en engineering term that describes a abnormal functioning of a part, running in a cause-generates-effect-generates-cause loop. For example, when current passes trough an electronic part some power is dissipated in the form of heat, equal to the part's electrical resistance times the square of the passing current. But the heat is increasing the part's resistance, so the power dissipated on it will increase, wich will make the part even hotter and making it dissipate even more power and so on, usually until either the part self-destructs or a protection system shuts it down. This is called "Thermal Runaway".

I am assuming Fugger noticed a point over wich the current drawn by the CPU increases rapidly because some transistors / junctions enter some kind of loop. There is one chance that it could be simple leakage (unwanted current flow trough theoretically-closed transistors), but i wouldn't bet on it since Intel's 45nm high-k process showed no such problems and the subzero temps should cancel them anyway pretty easy. The second guess, and my money is on this one, is that Nehalem's design is somewhat unfit for high voltages and over a certain value it just starts letting current pass trough it like there's no tomorrow. I have also noticed myself that over about 1.5V things stop scaling, from +50C all the way down to -80C, and this can only be happening due to electronic design.