Electronic load MOSFET drain to source current suddenly increases
There is nothing magic about the power darlington BJT; its even more prone to thermal runaway than is the power MOSFET.

With the heating being the base-collector region, just 1micro below the emitter-base region, the runaway timeconstant is just 11.4 nanoseconds or so.
(11.4 nS is the thermal tau of 1 cubic micron of silicon). With a fixed Vbe, and an internal-reducing Vbe of -2.2 milliVolts/degreeCentigrade, times 1 amp*5 volts (some power numbers), times 10 degreeCentrigrade/watt, times 4 milliVolts per 10% delta Ie (or Ic), we perturb these numbers (the 4milliVolts is our perturbation).

If a 4 millivolt perturbation produces more than 4mV output, you have a thermally unstable box (gain > +1 ).

What do we have here? 4mV => 10 % deltaI => 0.1 amp deltaI. DeltaPower is 5 volts * 0.1amp, or 0.5 watts. The deltaTemp is 0.5 watts * 10 degreeCent = 5 degreeCent. And knowing -2.2 milliVolts/degreeCent, we scale by 5, to get 11 milliVolts.

Since 11mV > 4mV, you would be thermall unstable. 

FETS have similar math, using different numbers.